package staticcheck

import (
	"fmt"
	"go/ast"
	"go/constant"
	"go/token"
	"go/types"
	htmltemplate "html/template"
	"net/http"
	"os"
	"reflect"
	"regexp"
	"regexp/syntax"
	"sort"
	"strconv"
	"strings"
	texttemplate "text/template"
	"unicode"

	"honnef.co/go/tools/analysis/code"
	"honnef.co/go/tools/analysis/edit"
	"honnef.co/go/tools/analysis/facts"
	"honnef.co/go/tools/analysis/facts/nilness"
	"honnef.co/go/tools/analysis/facts/typedness"
	"honnef.co/go/tools/analysis/lint"
	"honnef.co/go/tools/analysis/report"
	"honnef.co/go/tools/go/ast/astutil"
	"honnef.co/go/tools/go/ir"
	"honnef.co/go/tools/go/ir/irutil"
	"honnef.co/go/tools/go/types/typeutil"
	"honnef.co/go/tools/internal/passes/buildir"
	"honnef.co/go/tools/internal/sharedcheck"
	"honnef.co/go/tools/knowledge"
	"honnef.co/go/tools/pattern"
	"honnef.co/go/tools/printf"
	"honnef.co/go/tools/staticcheck/fakejson"
	"honnef.co/go/tools/staticcheck/fakereflect"
	"honnef.co/go/tools/staticcheck/fakexml"

	"golang.org/x/exp/typeparams"
	"golang.org/x/tools/go/analysis"
	"golang.org/x/tools/go/analysis/passes/inspect"
	"golang.org/x/tools/go/ast/inspector"
)

func checkSortSlice(call *Call) {
	c := call.Instr.Common().StaticCallee()
	arg := call.Args[0]

	T := arg.Value.Value.Type().Underlying()
	switch T.(type) {
	case *types.Interface:
		// we don't know.
		// TODO(dh): if the value is a phi node we can look at its edges
		if k, ok := arg.Value.Value.(*ir.Const); ok && k.Value == nil {
			// literal nil, e.g. sort.Sort(nil, ...)
			arg.Invalid(fmt.Sprintf("cannot call %s on nil literal", c))
		}
	case *types.Slice:
		// this is fine
	default:
		// this is not fine
		arg.Invalid(fmt.Sprintf("%s must only be called on slices, was called on %s", c, T))
	}
}

func validRegexp(call *Call) {
	arg := call.Args[0]
	err := ValidateRegexp(arg.Value)
	if err != nil {
		arg.Invalid(err.Error())
	}
}

type runeSlice []rune

func (rs runeSlice) Len() int               { return len(rs) }
func (rs runeSlice) Less(i int, j int) bool { return rs[i] < rs[j] }
func (rs runeSlice) Swap(i int, j int)      { rs[i], rs[j] = rs[j], rs[i] }

func utf8Cutset(call *Call) {
	arg := call.Args[1]
	if InvalidUTF8(arg.Value) {
		arg.Invalid(MsgInvalidUTF8)
	}
}

func uniqueCutset(call *Call) {
	arg := call.Args[1]
	if !UniqueStringCutset(arg.Value) {
		arg.Invalid(MsgNonUniqueCutset)
	}
}

func unmarshalPointer(name string, arg int) CallCheck {
	return func(call *Call) {
		if !Pointer(call.Args[arg].Value) {
			call.Args[arg].Invalid(fmt.Sprintf("%s expects to unmarshal into a pointer, but the provided value is not a pointer", name))
		}
	}
}

func pointlessIntMath(call *Call) {
	if ConvertedFromInt(call.Args[0].Value) {
		call.Invalid(fmt.Sprintf("calling %s on a converted integer is pointless", irutil.CallName(call.Instr.Common())))
	}
}

func checkValidHostPort(arg int) CallCheck {
	return func(call *Call) {
		if !ValidHostPort(call.Args[arg].Value) {
			call.Args[arg].Invalid(MsgInvalidHostPort)
		}
	}
}

var (
	checkRegexpRules = map[string]CallCheck{
		"regexp.MustCompile": validRegexp,
		"regexp.Compile":     validRegexp,
		"regexp.Match":       validRegexp,
		"regexp.MatchReader": validRegexp,
		"regexp.MatchString": validRegexp,
	}

	checkTimeParseRules = map[string]CallCheck{
		"time.Parse": func(call *Call) {
			arg := call.Args[knowledge.Arg("time.Parse.layout")]
			err := ValidateTimeLayout(arg.Value)
			if err != nil {
				arg.Invalid(err.Error())
			}
		},
	}

	checkEncodingBinaryRules = map[string]CallCheck{
		"encoding/binary.Write": func(call *Call) {
			arg := call.Args[knowledge.Arg("encoding/binary.Write.data")]
			if !CanBinaryMarshal(call.Pass, arg.Value) {
				arg.Invalid(fmt.Sprintf("value of type %s cannot be used with binary.Write", arg.Value.Value.Type()))
			}
		},
	}

	checkURLsRules = map[string]CallCheck{
		"net/url.Parse": func(call *Call) {
			arg := call.Args[knowledge.Arg("net/url.Parse.rawurl")]
			err := ValidateURL(arg.Value)
			if err != nil {
				arg.Invalid(err.Error())
			}
		},
	}

	checkSyncPoolValueRules = map[string]CallCheck{
		"(*sync.Pool).Put": func(call *Call) {
			arg := call.Args[knowledge.Arg("(*sync.Pool).Put.x")]
			typ := arg.Value.Value.Type()
			_, isSlice := typ.Underlying().(*types.Slice)
			if !typeutil.IsPointerLike(typ) || isSlice {
				arg.Invalid("argument should be pointer-like to avoid allocations")
			}
		},
	}

	checkRegexpFindAllRules = map[string]CallCheck{
		"(*regexp.Regexp).FindAll":                    RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllIndex":               RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllString":              RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllStringIndex":         RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllStringSubmatch":      RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllStringSubmatchIndex": RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllSubmatch":            RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllSubmatchIndex":       RepeatZeroTimes("a FindAll method", 1),
	}

	checkUTF8CutsetRules = map[string]CallCheck{
		"strings.IndexAny":     utf8Cutset,
		"strings.LastIndexAny": utf8Cutset,
		"strings.ContainsAny":  utf8Cutset,
		"strings.Trim":         utf8Cutset,
		"strings.TrimLeft":     utf8Cutset,
		"strings.TrimRight":    utf8Cutset,
	}

	checkUniqueCutsetRules = map[string]CallCheck{
		"strings.Trim":      uniqueCutset,
		"strings.TrimLeft":  uniqueCutset,
		"strings.TrimRight": uniqueCutset,
	}

	checkUnmarshalPointerRules = map[string]CallCheck{
		"encoding/xml.Unmarshal":                unmarshalPointer("xml.Unmarshal", 1),
		"(*encoding/xml.Decoder).Decode":        unmarshalPointer("Decode", 0),
		"(*encoding/xml.Decoder).DecodeElement": unmarshalPointer("DecodeElement", 0),
		"encoding/json.Unmarshal":               unmarshalPointer("json.Unmarshal", 1),
		"(*encoding/json.Decoder).Decode":       unmarshalPointer("Decode", 0),
	}

	checkUnbufferedSignalChanRules = map[string]CallCheck{
		"os/signal.Notify": func(call *Call) {
			arg := call.Args[knowledge.Arg("os/signal.Notify.c")]
			if UnbufferedChannel(arg.Value) {
				arg.Invalid("the channel used with signal.Notify should be buffered")
			}
		},
	}

	checkMathIntRules = map[string]CallCheck{
		"math.Ceil":  pointlessIntMath,
		"math.Floor": pointlessIntMath,
		"math.IsNaN": pointlessIntMath,
		"math.Trunc": pointlessIntMath,
		"math.IsInf": pointlessIntMath,
	}

	checkStringsReplaceZeroRules = map[string]CallCheck{
		"strings.Replace": RepeatZeroTimes("strings.Replace", 3),
		"bytes.Replace":   RepeatZeroTimes("bytes.Replace", 3),
	}

	checkListenAddressRules = map[string]CallCheck{
		"net/http.ListenAndServe":    checkValidHostPort(0),
		"net/http.ListenAndServeTLS": checkValidHostPort(0),
	}

	checkBytesEqualIPRules = map[string]CallCheck{
		"bytes.Equal": func(call *Call) {
			if ConvertedFrom(call.Args[knowledge.Arg("bytes.Equal.a")].Value, "net.IP") &&
				ConvertedFrom(call.Args[knowledge.Arg("bytes.Equal.b")].Value, "net.IP") {
				call.Invalid("use net.IP.Equal to compare net.IPs, not bytes.Equal")
			}
		},
	}

	checkRegexpMatchLoopRules = map[string]CallCheck{
		"regexp.Match":       loopedRegexp("regexp.Match"),
		"regexp.MatchReader": loopedRegexp("regexp.MatchReader"),
		"regexp.MatchString": loopedRegexp("regexp.MatchString"),
	}

	checkNoopMarshal = map[string]CallCheck{
		// TODO(dh): should we really flag XML? Even an empty struct
		// produces a non-zero amount of data, namely its type name.
		// Let's see if we encounter any false positives.
		//
		// Also, should we flag gob?
		"encoding/json.Marshal":           checkNoopMarshalImpl(knowledge.Arg("json.Marshal.v"), "MarshalJSON", "MarshalText"),
		"encoding/xml.Marshal":            checkNoopMarshalImpl(knowledge.Arg("xml.Marshal.v"), "MarshalXML", "MarshalText"),
		"(*encoding/json.Encoder).Encode": checkNoopMarshalImpl(knowledge.Arg("(*encoding/json.Encoder).Encode.v"), "MarshalJSON", "MarshalText"),
		"(*encoding/xml.Encoder).Encode":  checkNoopMarshalImpl(knowledge.Arg("(*encoding/xml.Encoder).Encode.v"), "MarshalXML", "MarshalText"),

		"encoding/json.Unmarshal":         checkNoopMarshalImpl(knowledge.Arg("json.Unmarshal.v"), "UnmarshalJSON", "UnmarshalText"),
		"encoding/xml.Unmarshal":          checkNoopMarshalImpl(knowledge.Arg("xml.Unmarshal.v"), "UnmarshalXML", "UnmarshalText"),
		"(*encoding/json.Decoder).Decode": checkNoopMarshalImpl(knowledge.Arg("(*encoding/json.Decoder).Decode.v"), "UnmarshalJSON", "UnmarshalText"),
		"(*encoding/xml.Decoder).Decode":  checkNoopMarshalImpl(knowledge.Arg("(*encoding/xml.Decoder).Decode.v"), "UnmarshalXML", "UnmarshalText"),
	}

	checkUnsupportedMarshal = map[string]CallCheck{
		"encoding/json.Marshal":           checkUnsupportedMarshalJSON,
		"encoding/xml.Marshal":            checkUnsupportedMarshalXML,
		"(*encoding/json.Encoder).Encode": checkUnsupportedMarshalJSON,
		"(*encoding/xml.Encoder).Encode":  checkUnsupportedMarshalXML,
	}

	checkAtomicAlignment = map[string]CallCheck{
		"sync/atomic.AddInt64":             checkAtomicAlignmentImpl,
		"sync/atomic.AddUint64":            checkAtomicAlignmentImpl,
		"sync/atomic.CompareAndSwapInt64":  checkAtomicAlignmentImpl,
		"sync/atomic.CompareAndSwapUint64": checkAtomicAlignmentImpl,
		"sync/atomic.LoadInt64":            checkAtomicAlignmentImpl,
		"sync/atomic.LoadUint64":           checkAtomicAlignmentImpl,
		"sync/atomic.StoreInt64":           checkAtomicAlignmentImpl,
		"sync/atomic.StoreUint64":          checkAtomicAlignmentImpl,
		"sync/atomic.SwapInt64":            checkAtomicAlignmentImpl,
		"sync/atomic.SwapUint64":           checkAtomicAlignmentImpl,
	}

	// TODO(dh): detect printf wrappers
	checkPrintfRules = map[string]CallCheck{
		"fmt.Errorf":                  func(call *Call) { checkPrintfCall(call, 0, 1) },
		"fmt.Printf":                  func(call *Call) { checkPrintfCall(call, 0, 1) },
		"fmt.Sprintf":                 func(call *Call) { checkPrintfCall(call, 0, 1) },
		"fmt.Fprintf":                 func(call *Call) { checkPrintfCall(call, 1, 2) },
		"golang.org/x/xerrors.Errorf": func(call *Call) { checkPrintfCall(call, 0, 1) },
	}

	checkSortSliceRules = map[string]CallCheck{
		"sort.Slice":         checkSortSlice,
		"sort.SliceIsSorted": checkSortSlice,
		"sort.SliceStable":   checkSortSlice,
	}

	checkWithValueKeyRules = map[string]CallCheck{
		"context.WithValue": checkWithValueKey,
	}

	checkStrconvRules = map[string]CallCheck{
		"strconv.ParseComplex": func(call *Call) {
			validateComplexBitSize(call.Args[knowledge.Arg("strconv.ParseComplex.bitSize")])
		},
		"strconv.ParseFloat": func(call *Call) {
			validateFloatBitSize(call.Args[knowledge.Arg("strconv.ParseFloat.bitSize")])
		},
		"strconv.ParseInt": func(call *Call) {
			validateContinuousBitSize(call.Args[knowledge.Arg("strconv.ParseInt.bitSize")], 0, 64)
			validateIntBaseAllowZero(call.Args[knowledge.Arg("strconv.ParseInt.base")])
		},
		"strconv.ParseUint": func(call *Call) {
			validateContinuousBitSize(call.Args[knowledge.Arg("strconv.ParseUint.bitSize")], 0, 64)
			validateIntBaseAllowZero(call.Args[knowledge.Arg("strconv.ParseUint.base")])
		},

		"strconv.FormatComplex": func(call *Call) {
			validateComplexFormat(call.Args[knowledge.Arg("strconv.FormatComplex.fmt")])
			validateComplexBitSize(call.Args[knowledge.Arg("strconv.FormatComplex.bitSize")])
		},
		"strconv.FormatFloat": func(call *Call) {
			validateFloatFormat(call.Args[knowledge.Arg("strconv.FormatFloat.fmt")])
			validateFloatBitSize(call.Args[knowledge.Arg("strconv.FormatFloat.bitSize")])
		},
		"strconv.FormatInt": func(call *Call) {
			validateIntBase(call.Args[knowledge.Arg("strconv.FormatInt.base")])
		},
		"strconv.FormatUint": func(call *Call) {
			validateIntBase(call.Args[knowledge.Arg("strconv.FormatUint.base")])
		},

		"strconv.AppendFloat": func(call *Call) {
			validateFloatFormat(call.Args[knowledge.Arg("strconv.AppendFloat.fmt")])
			validateFloatBitSize(call.Args[knowledge.Arg("strconv.AppendFloat.bitSize")])
		},
		"strconv.AppendInt": func(call *Call) {
			validateIntBase(call.Args[knowledge.Arg("strconv.AppendInt.base")])
		},
		"strconv.AppendUint": func(call *Call) {
			validateIntBase(call.Args[knowledge.Arg("strconv.AppendUint.base")])
		},
	}
)

func validateIntBase(arg *Argument) {
	if c := extractConstExpectKind(arg.Value.Value, constant.Int); c != nil {
		val, _ := constant.Int64Val(c.Value)
		if val < 2 {
			arg.Invalid("'base' must not be smaller than 2")
		}
		if val > 36 {
			arg.Invalid("'base' must not be larger than 36")
		}
	}
}

func validateIntBaseAllowZero(arg *Argument) {
	if c := extractConstExpectKind(arg.Value.Value, constant.Int); c != nil {
		val, _ := constant.Int64Val(c.Value)
		if val < 2 && val != 0 {
			arg.Invalid("'base' must not be smaller than 2, unless it is 0")
		}
		if val > 36 {
			arg.Invalid("'base' must not be larger than 36")
		}
	}
}

func validateComplexFormat(arg *Argument) {
	validateFloatFormat(arg)
}

func validateFloatFormat(arg *Argument) {
	if c := extractConstExpectKind(arg.Value.Value, constant.Int); c != nil {
		val, _ := constant.Int64Val(c.Value)
		switch val {
		case 'b', 'e', 'E', 'f', 'g', 'G', 'x', 'X':
		default:
			arg.Invalid(fmt.Sprintf("'fmt' argument is invalid: unknown format %q", val))
		}
	}
}

func validateComplexBitSize(arg *Argument) { validateDiscreetBitSize(arg, 64, 128) }
func validateFloatBitSize(arg *Argument)   { validateDiscreetBitSize(arg, 32, 64) }

func validateDiscreetBitSize(arg *Argument, size1 int, size2 int) {
	if c := extractConstExpectKind(arg.Value.Value, constant.Int); c != nil {
		val, _ := constant.Int64Val(c.Value)
		if val != int64(size1) && val != int64(size2) {
			arg.Invalid(fmt.Sprintf("'bitSize' argument is invalid, must be either %d or %d", size1, size2))
		}
	}
}

func validateContinuousBitSize(arg *Argument, min int, max int) {
	if c := extractConstExpectKind(arg.Value.Value, constant.Int); c != nil {
		val, _ := constant.Int64Val(c.Value)
		if val < int64(min) || val > int64(max) {
			arg.Invalid(fmt.Sprintf("'bitSize' argument is invalid, must be within %d and %d", min, max))
		}
	}
}

func checkPrintfCall(call *Call, fIdx, vIdx int) {
	f := call.Args[fIdx]
	var args []ir.Value
	switch v := call.Args[vIdx].Value.Value.(type) {
	case *ir.Slice:
		var ok bool
		args, ok = irutil.Vararg(v)
		if !ok {
			// We don't know what the actual arguments to the function are
			return
		}
	case *ir.Const:
		// nil, i.e. no arguments
	default:
		// We don't know what the actual arguments to the function are
		return
	}
	checkPrintfCallImpl(f, f.Value.Value, args)
}

type verbFlag int

const (
	isInt verbFlag = 1 << iota
	isBool
	isFP
	isString
	isPointer
	// Verbs that accept "pseudo pointers" will sometimes dereference
	// non-nil pointers. For example, %x on a non-nil *struct will print the
	// individual fields, but on a nil pointer it will print the address.
	isPseudoPointer
	isSlice
	isAny
	noRecurse
)

var verbs = [...]verbFlag{
	'b': isPseudoPointer | isInt | isFP,
	'c': isInt,
	'd': isPseudoPointer | isInt,
	'e': isFP,
	'E': isFP,
	'f': isFP,
	'F': isFP,
	'g': isFP,
	'G': isFP,
	'o': isPseudoPointer | isInt,
	'O': isPseudoPointer | isInt,
	'p': isSlice | isPointer | noRecurse,
	'q': isInt | isString,
	's': isString,
	't': isBool,
	'T': isAny,
	'U': isInt,
	'v': isAny,
	'X': isPseudoPointer | isInt | isFP | isString,
	'x': isPseudoPointer | isInt | isFP | isString,
}

func checkPrintfCallImpl(carg *Argument, f ir.Value, args []ir.Value) {
	var msCache *typeutil.MethodSetCache
	if f.Parent() != nil {
		msCache = &f.Parent().Prog.MethodSets
	}

	elem := func(T types.Type, verb rune) ([]types.Type, bool) {
		if verbs[verb]&noRecurse != 0 {
			return []types.Type{T}, false
		}
		switch T := T.(type) {
		case *types.Slice:
			if verbs[verb]&isSlice != 0 {
				return []types.Type{T}, false
			}
			if verbs[verb]&isString != 0 && typeutil.IsType(T.Elem().Underlying(), "byte") {
				return []types.Type{T}, false
			}
			return []types.Type{T.Elem()}, true
		case *types.Map:
			key := T.Key()
			val := T.Elem()
			return []types.Type{key, val}, true
		case *types.Struct:
			out := make([]types.Type, 0, T.NumFields())
			for i := 0; i < T.NumFields(); i++ {
				out = append(out, T.Field(i).Type())
			}
			return out, true
		case *types.Array:
			return []types.Type{T.Elem()}, true
		default:
			return []types.Type{T}, false
		}
	}
	isInfo := func(T types.Type, info types.BasicInfo) bool {
		basic, ok := T.Underlying().(*types.Basic)
		return ok && basic.Info()&info != 0
	}

	isStringer := func(T types.Type, ms *types.MethodSet) bool {
		sel := ms.Lookup(nil, "String")
		if sel == nil {
			return false
		}
		fn, ok := sel.Obj().(*types.Func)
		if !ok {
			// should be unreachable
			return false
		}
		sig := fn.Type().(*types.Signature)
		if sig.Params().Len() != 0 {
			return false
		}
		if sig.Results().Len() != 1 {
			return false
		}
		if !typeutil.IsType(sig.Results().At(0).Type(), "string") {
			return false
		}
		return true
	}
	isError := func(T types.Type, ms *types.MethodSet) bool {
		sel := ms.Lookup(nil, "Error")
		if sel == nil {
			return false
		}
		fn, ok := sel.Obj().(*types.Func)
		if !ok {
			// should be unreachable
			return false
		}
		sig := fn.Type().(*types.Signature)
		if sig.Params().Len() != 0 {
			return false
		}
		if sig.Results().Len() != 1 {
			return false
		}
		if !typeutil.IsType(sig.Results().At(0).Type(), "string") {
			return false
		}
		return true
	}

	isFormatter := func(T types.Type, ms *types.MethodSet) bool {
		sel := ms.Lookup(nil, "Format")
		if sel == nil {
			return false
		}
		fn, ok := sel.Obj().(*types.Func)
		if !ok {
			// should be unreachable
			return false
		}
		sig := fn.Type().(*types.Signature)
		if sig.Params().Len() != 2 {
			return false
		}
		// TODO(dh): check the types of the arguments for more
		// precision
		if sig.Results().Len() != 0 {
			return false
		}
		return true
	}

	seen := map[types.Type]bool{}
	var checkType func(verb rune, T types.Type, top bool) bool
	checkType = func(verb rune, T types.Type, top bool) bool {
		if top {
			for k := range seen {
				delete(seen, k)
			}
		}
		if seen[T] {
			return true
		}
		seen[T] = true
		if int(verb) >= len(verbs) {
			// Unknown verb
			return true
		}

		flags := verbs[verb]
		if flags == 0 {
			// Unknown verb
			return true
		}

		ms := msCache.MethodSet(T)
		if isFormatter(T, ms) {
			// the value is responsible for formatting itself
			return true
		}

		if flags&isString != 0 && (isStringer(T, ms) || isError(T, ms)) {
			// Check for stringer early because we're about to dereference
			return true
		}

		T = T.Underlying()
		if flags&(isPointer|isPseudoPointer) == 0 && top {
			T = typeutil.Dereference(T)
		}
		if flags&isPseudoPointer != 0 && top {
			t := typeutil.Dereference(T)
			if _, ok := t.Underlying().(*types.Struct); ok {
				T = t
			}
		}

		if _, ok := T.(*types.Interface); ok {
			// We don't know what's in the interface
			return true
		}

		var info types.BasicInfo
		if flags&isInt != 0 {
			info |= types.IsInteger
		}
		if flags&isBool != 0 {
			info |= types.IsBoolean
		}
		if flags&isFP != 0 {
			info |= types.IsFloat | types.IsComplex
		}
		if flags&isString != 0 {
			info |= types.IsString
		}

		if info != 0 && isInfo(T, info) {
			return true
		}

		if flags&isString != 0 {
			isStringyElem := func(typ types.Type) bool {
				if typ, ok := typ.Underlying().(*types.Basic); ok {
					return typ.Kind() == types.Byte
				}
				return false
			}
			switch T := T.(type) {
			case *types.Slice:
				if isStringyElem(T.Elem()) {
					return true
				}
			case *types.Array:
				if isStringyElem(T.Elem()) {
					return true
				}
			}
			if isStringer(T, ms) || isError(T, ms) {
				return true
			}
		}

		if flags&isPointer != 0 && typeutil.IsPointerLike(T) {
			return true
		}
		if flags&isPseudoPointer != 0 {
			switch U := T.Underlying().(type) {
			case *types.Pointer:
				if !top {
					return true
				}

				if _, ok := U.Elem().Underlying().(*types.Struct); !ok {
					// TODO(dh): can this condition ever be false? For
					// *T, if T is a struct, we'll already have
					// dereferenced it, meaning the *types.Pointer
					// branch couldn't have been taken. For T that
					// aren't structs, this condition will always
					// evaluate to true.
					return true
				}
			case *types.Chan, *types.Signature:
				// Channels and functions are always treated as
				// pointers and never recursed into.
				return true
			case *types.Basic:
				if U.Kind() == types.UnsafePointer {
					return true
				}
			case *types.Interface:
				// we will already have bailed if the type is an
				// interface.
				panic("unreachable")
			default:
				// other pointer-like types, such as maps or slices,
				// will be printed element-wise.
			}
		}

		if flags&isSlice != 0 {
			if _, ok := T.(*types.Slice); ok {
				return true
			}
		}

		if flags&isAny != 0 {
			return true
		}

		elems, ok := elem(T.Underlying(), verb)
		if !ok {
			return false
		}
		for _, elem := range elems {
			if !checkType(verb, elem, false) {
				return false
			}
		}

		return true
	}

	k, ok := irutil.Flatten(f).(*ir.Const)
	if !ok {
		return
	}
	actions, err := printf.Parse(constant.StringVal(k.Value))
	if err != nil {
		carg.Invalid("couldn't parse format string")
		return
	}

	ptr := 1
	hasExplicit := false

	checkStar := func(verb printf.Verb, star printf.Argument) bool {
		if star, ok := star.(printf.Star); ok {
			idx := 0
			if star.Index == -1 {
				idx = ptr
				ptr++
			} else {
				hasExplicit = true
				idx = star.Index
				ptr = star.Index + 1
			}
			if idx == 0 {
				carg.Invalid(fmt.Sprintf("Printf format %s reads invalid arg 0; indices are 1-based", verb.Raw))
				return false
			}
			if idx > len(args) {
				carg.Invalid(
					fmt.Sprintf("Printf format %s reads arg #%d, but call has only %d args",
						verb.Raw, idx, len(args)))
				return false
			}
			if arg, ok := args[idx-1].(*ir.MakeInterface); ok {
				if !isInfo(arg.X.Type(), types.IsInteger) {
					carg.Invalid(fmt.Sprintf("Printf format %s reads non-int arg #%d as argument of *", verb.Raw, idx))
				}
			}
		}
		return true
	}

	// We only report one problem per format string. Making a
	// mistake with an index tends to invalidate all future
	// implicit indices.
	for _, action := range actions {
		verb, ok := action.(printf.Verb)
		if !ok {
			continue
		}

		if !checkStar(verb, verb.Width) || !checkStar(verb, verb.Precision) {
			return
		}

		off := ptr
		if verb.Value != -1 {
			hasExplicit = true
			off = verb.Value
		}
		if off > len(args) {
			carg.Invalid(
				fmt.Sprintf("Printf format %s reads arg #%d, but call has only %d args",
					verb.Raw, off, len(args)))
			return
		} else if verb.Value == 0 && verb.Letter != '%' {
			carg.Invalid(fmt.Sprintf("Printf format %s reads invalid arg 0; indices are 1-based", verb.Raw))
			return
		} else if off != 0 {
			arg, ok := args[off-1].(*ir.MakeInterface)
			if ok {
				if !checkType(verb.Letter, arg.X.Type(), true) {
					carg.Invalid(fmt.Sprintf("Printf format %s has arg #%d of wrong type %s",
						verb.Raw, ptr, args[ptr-1].(*ir.MakeInterface).X.Type()))
					return
				}
			}
		}

		switch verb.Value {
		case -1:
			// Consume next argument
			ptr++
		case 0:
			// Don't consume any arguments
		default:
			ptr = verb.Value + 1
		}
	}

	if !hasExplicit && ptr <= len(args) {
		carg.Invalid(fmt.Sprintf("Printf call needs %d args but has %d args", ptr-1, len(args)))
	}
}

func checkAtomicAlignmentImpl(call *Call) {
	sizes := call.Pass.TypesSizes
	if sizes.Sizeof(types.Typ[types.Uintptr]) != 4 {
		// Not running on a 32-bit platform
		return
	}
	v, ok := irutil.Flatten(call.Args[0].Value.Value).(*ir.FieldAddr)
	if !ok {
		// TODO(dh): also check indexing into arrays and slices
		return
	}
	T := v.X.Type().Underlying().(*types.Pointer).Elem().Underlying().(*types.Struct)
	fields := make([]*types.Var, 0, T.NumFields())
	for i := 0; i < T.NumFields() && i <= v.Field; i++ {
		fields = append(fields, T.Field(i))
	}

	off := sizes.Offsetsof(fields)[v.Field]
	if off%8 != 0 {
		msg := fmt.Sprintf("address of non 64-bit aligned field %s passed to %s",
			T.Field(v.Field).Name(),
			irutil.CallName(call.Instr.Common()))
		call.Invalid(msg)
	}
}

func checkNoopMarshalImpl(argN int, meths ...string) CallCheck {
	return func(call *Call) {
		if code.IsGenerated(call.Pass, call.Instr.Pos()) {
			return
		}
		arg := call.Args[argN]
		T := arg.Value.Value.Type()
		Ts, ok := typeutil.Dereference(T).Underlying().(*types.Struct)
		if !ok {
			return
		}
		if Ts.NumFields() == 0 {
			return
		}
		fields := typeutil.FlattenFields(Ts)
		for _, field := range fields {
			if field.Var.Exported() {
				return
			}
		}
		// OPT(dh): we could use a method set cache here
		ms := call.Instr.Parent().Prog.MethodSets.MethodSet(T)
		// TODO(dh): we're not checking the signature, which can cause false negatives.
		// This isn't a huge problem, however, since vet complains about incorrect signatures.
		for _, meth := range meths {
			if ms.Lookup(nil, meth) != nil {
				return
			}
		}
		arg.Invalid(fmt.Sprintf("struct type '%s' doesn't have any exported fields, nor custom marshaling", typeutil.Dereference(T)))
	}
}

func checkUnsupportedMarshalJSON(call *Call) {
	arg := call.Args[0]
	T := arg.Value.Value.Type()
	if err := fakejson.Marshal(T); err != nil {
		typ := types.TypeString(err.Type, types.RelativeTo(arg.Value.Value.Parent().Pkg.Pkg))
		if err.Path == "x" {
			arg.Invalid(fmt.Sprintf("trying to marshal unsupported type %s", typ))
		} else {
			arg.Invalid(fmt.Sprintf("trying to marshal unsupported type %s, via %s", typ, err.Path))
		}
	}
}

func checkUnsupportedMarshalXML(call *Call) {
	arg := call.Args[0]
	T := arg.Value.Value.Type()
	if err := fakexml.Marshal(T); err != nil {
		switch err := err.(type) {
		case *fakexml.UnsupportedTypeError:
			typ := types.TypeString(err.Type, types.RelativeTo(arg.Value.Value.Parent().Pkg.Pkg))
			if err.Path == "x" {
				arg.Invalid(fmt.Sprintf("trying to marshal unsupported type %s", typ))
			} else {
				arg.Invalid(fmt.Sprintf("trying to marshal unsupported type %s, via %s", typ, err.Path))
			}
		case *fakexml.CyclicTypeError:
			typ := types.TypeString(err.Type, types.RelativeTo(arg.Value.Value.Parent().Pkg.Pkg))
			if err.Path == "x" {
				arg.Invalid(fmt.Sprintf("trying to marshal cyclic type %s", typ))
			} else {
				arg.Invalid(fmt.Sprintf("trying to marshal cyclic type %s, via %s", typ, err.Path))
			}
		case *fakexml.TagPathError:
			// Vet does a better job at reporting this error, because it can flag the actual struct tags, not just the call to Marshal
		default:
			// These errors get reported by SA5008 instead, which can flag the actual fields, independently of calls to xml.Marshal
		}
	}
}

func isInLoop(b *ir.BasicBlock) bool {
	sets := irutil.FindLoops(b.Parent())
	for _, set := range sets {
		if set.Has(b) {
			return true
		}
	}
	return false
}

func CheckUntrappableSignal(pass *analysis.Pass) (interface{}, error) {
	isSignal := func(pass *analysis.Pass, expr ast.Expr, name string) bool {
		if expr, ok := expr.(*ast.SelectorExpr); ok {
			return code.SelectorName(pass, expr) == name
		} else {
			return false
		}
	}

	fn := func(node ast.Node) {
		call := node.(*ast.CallExpr)
		if !code.IsCallToAny(pass, call,
			"os/signal.Ignore", "os/signal.Notify", "os/signal.Reset") {
			return
		}

		hasSigterm := false
		for _, arg := range call.Args {
			if conv, ok := arg.(*ast.CallExpr); ok && isSignal(pass, conv.Fun, "os.Signal") {
				arg = conv.Args[0]
			}

			if isSignal(pass, arg, "syscall.SIGTERM") {
				hasSigterm = true
				break
			}

		}
		for i, arg := range call.Args {
			if conv, ok := arg.(*ast.CallExpr); ok && isSignal(pass, conv.Fun, "os.Signal") {
				arg = conv.Args[0]
			}

			if isSignal(pass, arg, "os.Kill") || isSignal(pass, arg, "syscall.SIGKILL") {
				var fixes []analysis.SuggestedFix
				if !hasSigterm {
					nargs := make([]ast.Expr, len(call.Args))
					for j, a := range call.Args {
						if i == j {
							nargs[j] = edit.Selector("syscall", "SIGTERM")
						} else {
							nargs[j] = a
						}
					}
					ncall := *call
					ncall.Args = nargs
					fixes = append(fixes, edit.Fix(fmt.Sprintf("use syscall.SIGTERM instead of %s", report.Render(pass, arg)), edit.ReplaceWithNode(pass.Fset, call, &ncall)))
				}
				nargs := make([]ast.Expr, 0, len(call.Args))
				for j, a := range call.Args {
					if i == j {
						continue
					}
					nargs = append(nargs, a)
				}
				ncall := *call
				ncall.Args = nargs
				fixes = append(fixes, edit.Fix(fmt.Sprintf("remove %s from list of arguments", report.Render(pass, arg)), edit.ReplaceWithNode(pass.Fset, call, &ncall)))
				report.Report(pass, arg, fmt.Sprintf("%s cannot be trapped (did you mean syscall.SIGTERM?)", report.Render(pass, arg)), report.Fixes(fixes...))
			}
			if isSignal(pass, arg, "syscall.SIGSTOP") {
				nargs := make([]ast.Expr, 0, len(call.Args)-1)
				for j, a := range call.Args {
					if i == j {
						continue
					}
					nargs = append(nargs, a)
				}
				ncall := *call
				ncall.Args = nargs
				report.Report(pass, arg, "syscall.SIGSTOP cannot be trapped", report.Fixes(edit.Fix("remove syscall.SIGSTOP from list of arguments", edit.ReplaceWithNode(pass.Fset, call, &ncall))))
			}
		}
	}
	code.Preorder(pass, fn, (*ast.CallExpr)(nil))
	return nil, nil
}

func CheckTemplate(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		call := node.(*ast.CallExpr)
		// OPT(dh): use integer for kind
		var kind string
		switch code.CallName(pass, call) {
		case "(*text/template.Template).Parse":
			kind = "text"
		case "(*html/template.Template).Parse":
			kind = "html"
		default:
			return
		}
		sel := call.Fun.(*ast.SelectorExpr)
		if !code.IsCallToAny(pass, sel.X, "text/template.New", "html/template.New") {
			// TODO(dh): this is a cheap workaround for templates with
			// different delims. A better solution with less false
			// negatives would use data flow analysis to see where the
			// template comes from and where it has been
			return
		}
		s, ok := code.ExprToString(pass, call.Args[knowledge.Arg("(*text/template.Template).Parse.text")])
		if !ok {
			return
		}
		var err error
		switch kind {
		case "text":
			_, err = texttemplate.New("").Parse(s)
		case "html":
			_, err = htmltemplate.New("").Parse(s)
		}
		if err != nil {
			// TODO(dominikh): whitelist other parse errors, if any
			if strings.Contains(err.Error(), "unexpected") {
				report.Report(pass, call.Args[knowledge.Arg("(*text/template.Template).Parse.text")], err.Error())
			}
		}
	}
	code.Preorder(pass, fn, (*ast.CallExpr)(nil))
	return nil, nil
}

var (
	checkTimeSleepConstantPatternQ   = pattern.MustParse(`(CallExpr (Function "time.Sleep") lit@(IntegerLiteral value))`)
	checkTimeSleepConstantPatternRns = pattern.MustParse(`(BinaryExpr duration "*" (SelectorExpr (Ident "time") (Ident "Nanosecond")))`)
	checkTimeSleepConstantPatternRs  = pattern.MustParse(`(BinaryExpr duration "*" (SelectorExpr (Ident "time") (Ident "Second")))`)
)

func CheckTimeSleepConstant(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		m, ok := code.Match(pass, checkTimeSleepConstantPatternQ, node)
		if !ok {
			return
		}
		n, ok := constant.Int64Val(m.State["value"].(types.TypeAndValue).Value)
		if !ok {
			return
		}
		if n == 0 || n > 120 {
			// time.Sleep(0) is a seldom used pattern in concurrency
			// tests. >120 might be intentional. 120 was chosen
			// because the user could've meant 2 minutes.
			return
		}

		lit := m.State["lit"].(ast.Node)
		report.Report(pass, lit,
			fmt.Sprintf("sleeping for %d nanoseconds is probably a bug; be explicit if it isn't", n), report.Fixes(
				edit.Fix("explicitly use nanoseconds", edit.ReplaceWithPattern(pass.Fset, lit, checkTimeSleepConstantPatternRns, pattern.State{"duration": lit})),
				edit.Fix("use seconds", edit.ReplaceWithPattern(pass.Fset, lit, checkTimeSleepConstantPatternRs, pattern.State{"duration": lit}))))
	}
	code.Preorder(pass, fn, (*ast.CallExpr)(nil))
	return nil, nil
}

var checkWaitgroupAddQ = pattern.MustParse(`
	(GoStmt
		(CallExpr
			(FuncLit
				_
				call@(CallExpr (Function "(*sync.WaitGroup).Add") _):_) _))`)

func CheckWaitgroupAdd(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		if m, ok := code.Match(pass, checkWaitgroupAddQ, node); ok {
			call := m.State["call"].(ast.Node)
			report.Report(pass, call, fmt.Sprintf("should call %s before starting the goroutine to avoid a race", report.Render(pass, call)))
		}
	}
	code.Preorder(pass, fn, (*ast.GoStmt)(nil))
	return nil, nil
}

func CheckInfiniteEmptyLoop(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		loop := node.(*ast.ForStmt)
		if len(loop.Body.List) != 0 || loop.Post != nil {
			return
		}

		if loop.Init != nil {
			// TODO(dh): this isn't strictly necessary, it just makes
			// the check easier.
			return
		}
		// An empty loop is bad news in two cases: 1) The loop has no
		// condition. In that case, it's just a loop that spins
		// forever and as fast as it can, keeping a core busy. 2) The
		// loop condition only consists of variable or field reads and
		// operators on those. The only way those could change their
		// value is with unsynchronised access, which constitutes a
		// data race.
		//
		// If the condition contains any function calls, its behaviour
		// is dynamic and the loop might terminate. Similarly for
		// channel receives.

		if loop.Cond != nil {
			if code.MayHaveSideEffects(pass, loop.Cond, nil) {
				return
			}
			if ident, ok := loop.Cond.(*ast.Ident); ok {
				if k, ok := pass.TypesInfo.ObjectOf(ident).(*types.Const); ok {
					if !constant.BoolVal(k.Val()) {
						// don't flag `for false {}` loops. They're a debug aid.
						return
					}
				}
			}
			report.Report(pass, loop, "loop condition never changes or has a race condition")
		}
		report.Report(pass, loop, "this loop will spin, using 100% CPU", report.ShortRange())
	}
	code.Preorder(pass, fn, (*ast.ForStmt)(nil))
	return nil, nil
}

func CheckDeferInInfiniteLoop(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		mightExit := false
		var defers []ast.Stmt
		loop := node.(*ast.ForStmt)
		if loop.Cond != nil {
			return
		}
		fn2 := func(node ast.Node) bool {
			switch stmt := node.(type) {
			case *ast.ReturnStmt:
				mightExit = true
				return false
			case *ast.BranchStmt:
				// TODO(dominikh): if this sees a break in a switch or
				// select, it doesn't check if it breaks the loop or
				// just the select/switch. This causes some false
				// negatives.
				if stmt.Tok == token.BREAK {
					mightExit = true
					return false
				}
			case *ast.DeferStmt:
				defers = append(defers, stmt)
			case *ast.FuncLit:
				// Don't look into function bodies
				return false
			}
			return true
		}
		ast.Inspect(loop.Body, fn2)
		if mightExit {
			return
		}
		for _, stmt := range defers {
			report.Report(pass, stmt, "defers in this infinite loop will never run")
		}
	}
	code.Preorder(pass, fn, (*ast.ForStmt)(nil))
	return nil, nil
}

func CheckDubiousDeferInChannelRangeLoop(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		loop := node.(*ast.RangeStmt)
		typ := pass.TypesInfo.TypeOf(loop.X)
		_, ok := typeutil.CoreType(typ).(*types.Chan)
		if !ok {
			return
		}
		fn2 := func(node ast.Node) bool {
			switch stmt := node.(type) {
			case *ast.DeferStmt:
				report.Report(pass, stmt, "defers in this range loop won't run unless the channel gets closed")
			case *ast.FuncLit:
				// Don't look into function bodies
				return false
			}
			return true
		}
		ast.Inspect(loop.Body, fn2)
	}
	code.Preorder(pass, fn, (*ast.RangeStmt)(nil))
	return nil, nil
}

func CheckTestMainExit(pass *analysis.Pass) (interface{}, error) {
	if code.IsGoVersion(pass, 15) {
		// Beginning with Go 1.15, the test framework will call
		// os.Exit for us.
		return nil, nil
	}

	var (
		fnmain    ast.Node
		callsExit bool
		callsRun  bool
		arg       types.Object
	)
	fn := func(node ast.Node, push bool) bool {
		if !push {
			if fnmain != nil && node == fnmain {
				if !callsExit && callsRun {
					report.Report(pass, fnmain, "TestMain should call os.Exit to set exit code")
				}
				fnmain = nil
				callsExit = false
				callsRun = false
				arg = nil
			}
			return true
		}

		switch node := node.(type) {
		case *ast.FuncDecl:
			if fnmain != nil {
				return true
			}
			if !isTestMain(pass, node) {
				return false
			}
			fnmain = node
			arg = pass.TypesInfo.ObjectOf(node.Type.Params.List[0].Names[0])
			return true
		case *ast.CallExpr:
			if code.IsCallTo(pass, node, "os.Exit") {
				callsExit = true
				return false
			}
			sel, ok := node.Fun.(*ast.SelectorExpr)
			if !ok {
				return true
			}
			ident, ok := sel.X.(*ast.Ident)
			if !ok {
				return true
			}
			if arg != pass.TypesInfo.ObjectOf(ident) {
				return true
			}
			if sel.Sel.Name == "Run" {
				callsRun = true
				return false
			}
			return true
		default:
			lint.ExhaustiveTypeSwitch(node)
			return true
		}
	}
	pass.ResultOf[inspect.Analyzer].(*inspector.Inspector).Nodes([]ast.Node{(*ast.FuncDecl)(nil), (*ast.CallExpr)(nil)}, fn)
	return nil, nil
}

func isTestMain(pass *analysis.Pass, decl *ast.FuncDecl) bool {
	if decl.Name.Name != "TestMain" {
		return false
	}
	if len(decl.Type.Params.List) != 1 {
		return false
	}
	arg := decl.Type.Params.List[0]
	if len(arg.Names) != 1 {
		return false
	}
	return code.IsOfType(pass, arg.Type, "*testing.M")
}

func CheckExec(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		call := node.(*ast.CallExpr)
		if !code.IsCallTo(pass, call, "os/exec.Command") {
			return
		}
		val, ok := code.ExprToString(pass, call.Args[knowledge.Arg("os/exec.Command.name")])
		if !ok {
			return
		}
		if !strings.Contains(val, " ") || strings.Contains(val, `\`) || strings.Contains(val, "/") {
			return
		}
		report.Report(pass, call.Args[knowledge.Arg("os/exec.Command.name")],
			"first argument to exec.Command looks like a shell command, but a program name or path are expected")
	}
	code.Preorder(pass, fn, (*ast.CallExpr)(nil))
	return nil, nil
}

func CheckLoopEmptyDefault(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		loop := node.(*ast.ForStmt)
		if len(loop.Body.List) != 1 || loop.Cond != nil || loop.Init != nil {
			return
		}
		sel, ok := loop.Body.List[0].(*ast.SelectStmt)
		if !ok {
			return
		}
		for _, c := range sel.Body.List {
			// FIXME this leaves behind an empty line, and possibly
			// comments in the default branch. We can't easily fix
			// either.
			if comm, ok := c.(*ast.CommClause); ok && comm.Comm == nil && len(comm.Body) == 0 {
				report.Report(pass, comm, "should not have an empty default case in a for+select loop; the loop will spin",
					report.Fixes(edit.Fix("remove empty default branch", edit.Delete(comm))))
				// there can only be one default case
				break
			}
		}
	}
	code.Preorder(pass, fn, (*ast.ForStmt)(nil))
	return nil, nil
}

func CheckLhsRhsIdentical(pass *analysis.Pass) (interface{}, error) {
	var isFloat func(T types.Type) bool
	isFloat = func(T types.Type) bool {
		tset := typeutil.NewTypeSet(T)
		if len(tset.Terms) == 0 {
			// no terms, so floats are a possibility
			return true
		}
		return tset.Any(func(term *typeparams.Term) bool {
			switch typ := term.Type().Underlying().(type) {
			case *types.Basic:
				kind := typ.Kind()
				return kind == types.Float32 || kind == types.Float64
			case *types.Array:
				return isFloat(typ.Elem())
			case *types.Struct:
				for i := 0; i < typ.NumFields(); i++ {
					if !isFloat(typ.Field(i).Type()) {
						return false
					}
				}
				return true
			default:
				return false
			}
		})
	}

	// TODO(dh): this check ignores the existence of side-effects and
	// happily flags fn() == fn() – so far, we've had nobody complain
	// about a false positive, and it's caught several bugs in real
	// code.
	//
	// We special case functions from the math/rand package. Someone ran
	// into the following false positive: "rand.Intn(2) - rand.Intn(2), which I wrote to generate values {-1, 0, 1} with {0.25, 0.5, 0.25} probability."
	fn := func(node ast.Node) {
		op := node.(*ast.BinaryExpr)
		switch op.Op {
		case token.EQL, token.NEQ:
		case token.SUB, token.QUO, token.AND, token.REM, token.OR, token.XOR, token.AND_NOT,
			token.LAND, token.LOR, token.LSS, token.GTR, token.LEQ, token.GEQ:
		default:
			// For some ops, such as + and *, it can make sense to
			// have identical operands
			return
		}

		if isFloat(pass.TypesInfo.TypeOf(op.X)) {
			// 'float <op> float' makes sense for several operators.
			// We've tried keeping an exact list of operators to allow, but floats keep surprising us. Let's just give up instead.
			return
		}

		if reflect.TypeOf(op.X) != reflect.TypeOf(op.Y) {
			return
		}
		if report.Render(pass, op.X) != report.Render(pass, op.Y) {
			return
		}
		l1, ok1 := op.X.(*ast.BasicLit)
		l2, ok2 := op.Y.(*ast.BasicLit)
		if ok1 && ok2 && l1.Kind == token.INT && l2.Kind == l1.Kind && l1.Value == "0" && l2.Value == l1.Value && code.IsGenerated(pass, l1.Pos()) {
			// cgo generates the following function call:
			// _cgoCheckPointer(_cgoBase0, 0 == 0) – it uses 0 == 0
			// instead of true in case the user shadowed the
			// identifier. Ideally we'd restrict this exception to
			// calls of _cgoCheckPointer, but it's not worth the
			// hassle of keeping track of the stack. <lit> <op> <lit>
			// are very rare to begin with, and we're mostly checking
			// for them to catch typos such as 1 == 1 where the user
			// meant to type i == 1. The odds of a false negative for
			// 0 == 0 are slim.
			return
		}

		if expr, ok := op.X.(*ast.CallExpr); ok {
			call := code.CallName(pass, expr)
			switch call {
			case "math/rand.Int",
				"math/rand.Int31",
				"math/rand.Int31n",
				"math/rand.Int63",
				"math/rand.Int63n",
				"math/rand.Intn",
				"math/rand.Uint32",
				"math/rand.Uint64",
				"math/rand.ExpFloat64",
				"math/rand.Float32",
				"math/rand.Float64",
				"math/rand.NormFloat64",
				"(*math/rand.Rand).Int",
				"(*math/rand.Rand).Int31",
				"(*math/rand.Rand).Int31n",
				"(*math/rand.Rand).Int63",
				"(*math/rand.Rand).Int63n",
				"(*math/rand.Rand).Intn",
				"(*math/rand.Rand).Uint32",
				"(*math/rand.Rand).Uint64",
				"(*math/rand.Rand).ExpFloat64",
				"(*math/rand.Rand).Float32",
				"(*math/rand.Rand).Float64",
				"(*math/rand.Rand).NormFloat64":
				return
			}
		}

		report.Report(pass, op, fmt.Sprintf("identical expressions on the left and right side of the '%s' operator", op.Op))
	}
	code.Preorder(pass, fn, (*ast.BinaryExpr)(nil))
	return nil, nil
}

func CheckScopedBreak(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		var body *ast.BlockStmt
		switch node := node.(type) {
		case *ast.ForStmt:
			body = node.Body
		case *ast.RangeStmt:
			body = node.Body
		default:
			lint.ExhaustiveTypeSwitch(node)
		}
		for _, stmt := range body.List {
			var blocks [][]ast.Stmt
			switch stmt := stmt.(type) {
			case *ast.SwitchStmt:
				for _, c := range stmt.Body.List {
					blocks = append(blocks, c.(*ast.CaseClause).Body)
				}
			case *ast.SelectStmt:
				for _, c := range stmt.Body.List {
					blocks = append(blocks, c.(*ast.CommClause).Body)
				}
			default:
				continue
			}

			for _, body := range blocks {
				if len(body) == 0 {
					continue
				}
				lasts := []ast.Stmt{body[len(body)-1]}
				// TODO(dh): unfold all levels of nested block
				// statements, not just a single level if statement
				if ifs, ok := lasts[0].(*ast.IfStmt); ok {
					if len(ifs.Body.List) == 0 {
						continue
					}
					lasts[0] = ifs.Body.List[len(ifs.Body.List)-1]

					if block, ok := ifs.Else.(*ast.BlockStmt); ok {
						if len(block.List) != 0 {
							lasts = append(lasts, block.List[len(block.List)-1])
						}
					}
				}
				for _, last := range lasts {
					branch, ok := last.(*ast.BranchStmt)
					if !ok || branch.Tok != token.BREAK || branch.Label != nil {
						continue
					}
					report.Report(pass, branch, "ineffective break statement. Did you mean to break out of the outer loop?")
				}
			}
		}
	}
	code.Preorder(pass, fn, (*ast.ForStmt)(nil), (*ast.RangeStmt)(nil))
	return nil, nil
}

func CheckUnsafePrintf(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		call := node.(*ast.CallExpr)
		name := code.CallName(pass, call)
		var arg int

		switch name {
		case "fmt.Printf", "fmt.Sprintf", "log.Printf":
			arg = knowledge.Arg("fmt.Printf.format")
		case "fmt.Fprintf":
			arg = knowledge.Arg("fmt.Fprintf.format")
		default:
			return
		}
		if len(call.Args) != arg+1 {
			return
		}
		switch call.Args[arg].(type) {
		case *ast.CallExpr, *ast.Ident:
		default:
			return
		}

		if _, ok := pass.TypesInfo.TypeOf(call.Args[arg]).(*types.Tuple); ok {
			// the called function returns multiple values and got
			// splatted into the call. for all we know, it is
			// returning good arguments.
			return
		}

		alt := name[:len(name)-1]
		report.Report(pass, call,
			"printf-style function with dynamic format string and no further arguments should use print-style function instead",
			report.Fixes(edit.Fix(fmt.Sprintf("use %s instead of %s", alt, name), edit.ReplaceWithString(call.Fun, alt))))
	}
	code.Preorder(pass, fn, (*ast.CallExpr)(nil))
	return nil, nil
}

func CheckEarlyDefer(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		block := node.(*ast.BlockStmt)
		if len(block.List) < 2 {
			return
		}
		for i, stmt := range block.List {
			if i == len(block.List)-1 {
				break
			}
			assign, ok := stmt.(*ast.AssignStmt)
			if !ok {
				continue
			}
			if len(assign.Rhs) != 1 {
				continue
			}
			if len(assign.Lhs) < 2 {
				continue
			}
			if lhs, ok := assign.Lhs[len(assign.Lhs)-1].(*ast.Ident); ok && lhs.Name == "_" {
				continue
			}
			call, ok := assign.Rhs[0].(*ast.CallExpr)
			if !ok {
				continue
			}
			sig, ok := pass.TypesInfo.TypeOf(call.Fun).(*types.Signature)
			if !ok {
				continue
			}
			if sig.Results().Len() < 2 {
				continue
			}
			last := sig.Results().At(sig.Results().Len() - 1)
			// FIXME(dh): check that it's error from universe, not
			// another type of the same name
			if last.Type().String() != "error" {
				continue
			}
			lhs, ok := assign.Lhs[0].(*ast.Ident)
			if !ok {
				continue
			}
			def, ok := block.List[i+1].(*ast.DeferStmt)
			if !ok {
				continue
			}
			sel, ok := def.Call.Fun.(*ast.SelectorExpr)
			if !ok {
				continue
			}
			ident, ok := selectorX(sel).(*ast.Ident)
			if !ok {
				continue
			}
			if ident.Obj != lhs.Obj {
				continue
			}
			if sel.Sel.Name != "Close" {
				continue
			}
			report.Report(pass, def, fmt.Sprintf("should check returned error before deferring %s", report.Render(pass, def.Call)))
		}
	}
	code.Preorder(pass, fn, (*ast.BlockStmt)(nil))
	return nil, nil
}

func selectorX(sel *ast.SelectorExpr) ast.Node {
	switch x := sel.X.(type) {
	case *ast.SelectorExpr:
		return selectorX(x)
	default:
		return x
	}
}

func CheckEmptyCriticalSection(pass *analysis.Pass) (interface{}, error) {
	if pass.Pkg.Path() == "sync_test" {
		// exception for the sync package's tests
		return nil, nil
	}

	// Initially it might seem like this check would be easier to
	// implement using IR. After all, we're only checking for two
	// consecutive method calls. In reality, however, there may be any
	// number of other instructions between the lock and unlock, while
	// still constituting an empty critical section. For example,
	// given `m.x().Lock(); m.x().Unlock()`, there will be a call to
	// x(). In the AST-based approach, this has a tiny potential for a
	// false positive (the second call to x might be doing work that
	// is protected by the mutex). In an IR-based approach, however,
	// it would miss a lot of real bugs.

	mutexParams := func(s ast.Stmt) (x ast.Expr, funcName string, ok bool) {
		expr, ok := s.(*ast.ExprStmt)
		if !ok {
			return nil, "", false
		}
		call, ok := expr.X.(*ast.CallExpr)
		if !ok {
			return nil, "", false
		}
		sel, ok := call.Fun.(*ast.SelectorExpr)
		if !ok {
			return nil, "", false
		}

		fn, ok := pass.TypesInfo.ObjectOf(sel.Sel).(*types.Func)
		if !ok {
			return nil, "", false
		}
		sig := fn.Type().(*types.Signature)
		if sig.Params().Len() != 0 || sig.Results().Len() != 0 {
			return nil, "", false
		}

		return sel.X, fn.Name(), true
	}

	fn := func(node ast.Node) {
		block := node.(*ast.BlockStmt)
		if len(block.List) < 2 {
			return
		}
		for i := range block.List[:len(block.List)-1] {
			sel1, method1, ok1 := mutexParams(block.List[i])
			sel2, method2, ok2 := mutexParams(block.List[i+1])

			if !ok1 || !ok2 || report.Render(pass, sel1) != report.Render(pass, sel2) {
				continue
			}
			if (method1 == "Lock" && method2 == "Unlock") ||
				(method1 == "RLock" && method2 == "RUnlock") {
				report.Report(pass, block.List[i+1], "empty critical section")
			}
		}
	}
	code.Preorder(pass, fn, (*ast.BlockStmt)(nil))
	return nil, nil
}

var (
	// cgo produces code like fn(&*_Cvar_kSomeCallbacks) which we don't
	// want to flag.
	cgoIdent               = regexp.MustCompile(`^_C(func|var)_.+$`)
	checkIneffectiveCopyQ1 = pattern.MustParse(`(UnaryExpr "&" (StarExpr obj))`)
	checkIneffectiveCopyQ2 = pattern.MustParse(`(StarExpr (UnaryExpr "&" _))`)
)

func CheckIneffectiveCopy(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		if m, ok := code.Match(pass, checkIneffectiveCopyQ1, node); ok {
			if ident, ok := m.State["obj"].(*ast.Ident); !ok || !cgoIdent.MatchString(ident.Name) {
				report.Report(pass, node, "&*x will be simplified to x. It will not copy x.")
			}
		} else if _, ok := code.Match(pass, checkIneffectiveCopyQ2, node); ok {
			report.Report(pass, node, "*&x will be simplified to x. It will not copy x.")
		}
	}
	code.Preorder(pass, fn, (*ast.UnaryExpr)(nil), (*ast.StarExpr)(nil))
	return nil, nil
}

func CheckCanonicalHeaderKey(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node, push bool) bool {
		if !push {
			return false
		}
		if assign, ok := node.(*ast.AssignStmt); ok {
			// TODO(dh): This risks missing some Header reads, for
			// example in `h1["foo"] = h2["foo"]` – these edge
			// cases are probably rare enough to ignore for now.
			for _, expr := range assign.Lhs {
				op, ok := expr.(*ast.IndexExpr)
				if !ok {
					continue
				}
				if code.IsOfType(pass, op.X, "net/http.Header") {
					return false
				}
			}
			return true
		}
		op, ok := node.(*ast.IndexExpr)
		if !ok {
			return true
		}
		if !code.IsOfType(pass, op.X, "net/http.Header") {
			return true
		}
		s, ok := code.ExprToString(pass, op.Index)
		if !ok {
			return true
		}
		canonical := http.CanonicalHeaderKey(s)
		if s == canonical {
			return true
		}
		var fix analysis.SuggestedFix
		switch op.Index.(type) {
		case *ast.BasicLit:
			fix = edit.Fix("canonicalize header key", edit.ReplaceWithString(op.Index, strconv.Quote(canonical)))
		case *ast.Ident:
			call := &ast.CallExpr{
				Fun:  edit.Selector("http", "CanonicalHeaderKey"),
				Args: []ast.Expr{op.Index},
			}
			fix = edit.Fix("wrap in http.CanonicalHeaderKey", edit.ReplaceWithNode(pass.Fset, op.Index, call))
		}
		msg := fmt.Sprintf("keys in http.Header are canonicalized, %q is not canonical; fix the constant or use http.CanonicalHeaderKey", s)
		if fix.Message != "" {
			report.Report(pass, op, msg, report.Fixes(fix))
		} else {
			report.Report(pass, op, msg)
		}
		return true
	}
	pass.ResultOf[inspect.Analyzer].(*inspector.Inspector).Nodes([]ast.Node{(*ast.AssignStmt)(nil), (*ast.IndexExpr)(nil)}, fn)
	return nil, nil
}

func CheckBenchmarkN(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		assign := node.(*ast.AssignStmt)
		if len(assign.Lhs) != 1 || len(assign.Rhs) != 1 {
			return
		}
		sel, ok := assign.Lhs[0].(*ast.SelectorExpr)
		if !ok {
			return
		}
		if sel.Sel.Name != "N" {
			return
		}
		if !code.IsOfType(pass, sel.X, "*testing.B") {
			return
		}
		report.Report(pass, assign, fmt.Sprintf("should not assign to %s", report.Render(pass, sel)))
	}
	code.Preorder(pass, fn, (*ast.AssignStmt)(nil))
	return nil, nil
}

func CheckUnreadVariableValues(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		if irutil.IsExample(fn) {
			continue
		}
		node := fn.Source()
		if node == nil {
			continue
		}
		if gen, ok := code.Generator(pass, node.Pos()); ok && gen == facts.Goyacc {
			// Don't flag unused values in code generated by goyacc.
			// There may be hundreds of those due to the way the state
			// machine is constructed.
			continue
		}

		switchTags := map[ir.Value]struct{}{}
		ast.Inspect(node, func(node ast.Node) bool {
			s, ok := node.(*ast.SwitchStmt)
			if !ok {
				return true
			}
			v, _ := fn.ValueForExpr(s.Tag)
			switchTags[v] = struct{}{}
			return true
		})

		// OPT(dh): don't use a map, possibly use a bitset
		var hasUse func(v ir.Value, seen map[ir.Value]struct{}) bool
		hasUse = func(v ir.Value, seen map[ir.Value]struct{}) bool {
			if _, ok := seen[v]; ok {
				return false
			}
			if _, ok := switchTags[v]; ok {
				return true
			}
			refs := v.Referrers()
			if refs == nil {
				// TODO investigate why refs can be nil
				return true
			}
			for _, ref := range *refs {
				switch ref := ref.(type) {
				case *ir.DebugRef:
				case *ir.Sigma:
					if seen == nil {
						seen = map[ir.Value]struct{}{}
					}
					seen[v] = struct{}{}
					if hasUse(ref, seen) {
						return true
					}
				case *ir.Phi:
					if seen == nil {
						seen = map[ir.Value]struct{}{}
					}
					seen[v] = struct{}{}
					if hasUse(ref, seen) {
						return true
					}
				default:
					return true
				}
			}
			return false
		}

		ast.Inspect(node, func(node ast.Node) bool {
			assign, ok := node.(*ast.AssignStmt)
			if !ok {
				return true
			}
			if len(assign.Lhs) > 1 && len(assign.Rhs) == 1 {
				// Either a function call with multiple return values,
				// or a comma-ok assignment

				val, _ := fn.ValueForExpr(assign.Rhs[0])
				if val == nil {
					return true
				}
				refs := val.Referrers()
				if refs == nil {
					return true
				}
				for _, ref := range *refs {
					ex, ok := ref.(*ir.Extract)
					if !ok {
						continue
					}
					if !hasUse(ex, nil) {
						lhs := assign.Lhs[ex.Index]
						if ident, ok := lhs.(*ast.Ident); !ok || ok && ident.Name == "_" {
							continue
						}
						report.Report(pass, assign, fmt.Sprintf("this value of %s is never used", lhs))
					}
				}
				return true
			}
			for i, lhs := range assign.Lhs {
				rhs := assign.Rhs[i]
				if ident, ok := lhs.(*ast.Ident); !ok || ok && ident.Name == "_" {
					continue
				}
				val, _ := fn.ValueForExpr(rhs)
				if val == nil {
					continue
				}

				if _, ok := val.(*ir.Const); ok {
					// a zero-valued constant, for example in 'foo := []string(nil)'
					continue
				}
				if !hasUse(val, nil) {
					report.Report(pass, assign, fmt.Sprintf("this value of %s is never used", lhs))
				}
			}
			return true
		})
	}
	return nil, nil
}

func CheckPredeterminedBooleanExprs(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, block := range fn.Blocks {
			for _, ins := range block.Instrs {
				binop, ok := ins.(*ir.BinOp)
				if !ok {
					continue
				}
				switch binop.Op {
				case token.GTR, token.LSS, token.EQL, token.NEQ, token.LEQ, token.GEQ:
				default:
					continue
				}

				xs, ok1 := consts(binop.X, nil, nil)
				ys, ok2 := consts(binop.Y, nil, nil)
				if !ok1 || !ok2 || len(xs) == 0 || len(ys) == 0 {
					continue
				}

				trues := 0
				for _, x := range xs {
					for _, y := range ys {
						if x.Value == nil {
							if y.Value == nil {
								trues++
							}
							continue
						}
						if constant.Compare(x.Value, binop.Op, y.Value) {
							trues++
						}
					}
				}
				b := trues != 0
				if trues == 0 || trues == len(xs)*len(ys) {
					report.Report(pass, binop, fmt.Sprintf("binary expression is always %t for all possible values (%s %s %s)", b, xs, binop.Op, ys))
				}
			}
		}
	}
	return nil, nil
}

func CheckNilMaps(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, block := range fn.Blocks {
			for _, ins := range block.Instrs {
				mu, ok := ins.(*ir.MapUpdate)
				if !ok {
					continue
				}
				c, ok := irutil.Flatten(mu.Map).(*ir.Const)
				if !ok {
					continue
				}
				if c.Value != nil {
					continue
				}
				report.Report(pass, mu, "assignment to nil map")
			}
		}
	}
	return nil, nil
}

func CheckExtremeComparison(pass *analysis.Pass) (interface{}, error) {
	isobj := func(expr ast.Expr, name string) bool {
		sel, ok := expr.(*ast.SelectorExpr)
		if !ok {
			return false
		}
		return typeutil.IsObject(pass.TypesInfo.ObjectOf(sel.Sel), name)
	}

	fn := func(node ast.Node) {
		expr := node.(*ast.BinaryExpr)
		tx := pass.TypesInfo.TypeOf(expr.X)
		basic, ok := tx.Underlying().(*types.Basic)
		if !ok {
			return
		}

		var max string
		var min string

		switch basic.Kind() {
		case types.Uint8:
			max = "math.MaxUint8"
		case types.Uint16:
			max = "math.MaxUint16"
		case types.Uint32:
			max = "math.MaxUint32"
		case types.Uint64:
			max = "math.MaxUint64"
		case types.Uint:
			max = "math.MaxUint64"

		case types.Int8:
			min = "math.MinInt8"
			max = "math.MaxInt8"
		case types.Int16:
			min = "math.MinInt16"
			max = "math.MaxInt16"
		case types.Int32:
			min = "math.MinInt32"
			max = "math.MaxInt32"
		case types.Int64:
			min = "math.MinInt64"
			max = "math.MaxInt64"
		case types.Int:
			min = "math.MinInt64"
			max = "math.MaxInt64"
		}

		if (expr.Op == token.GTR || expr.Op == token.GEQ) && isobj(expr.Y, max) ||
			(expr.Op == token.LSS || expr.Op == token.LEQ) && isobj(expr.X, max) {
			report.Report(pass, expr, fmt.Sprintf("no value of type %s is greater than %s", basic, max))
		}
		if expr.Op == token.LEQ && isobj(expr.Y, max) ||
			expr.Op == token.GEQ && isobj(expr.X, max) {
			report.Report(pass, expr, fmt.Sprintf("every value of type %s is <= %s", basic, max))
		}

		isZeroLiteral := func(expr ast.Expr) bool {
			return code.IsIntegerLiteral(pass, expr, constant.MakeInt64(0))
		}
		if (basic.Info() & types.IsUnsigned) != 0 {
			if (expr.Op == token.LSS && isZeroLiteral(expr.Y)) ||
				(expr.Op == token.GTR && isZeroLiteral(expr.X)) {
				report.Report(pass, expr, fmt.Sprintf("no value of type %s is less than 0", basic))
			}
			if expr.Op == token.GEQ && isZeroLiteral(expr.Y) ||
				expr.Op == token.LEQ && isZeroLiteral(expr.X) {
				report.Report(pass, expr, fmt.Sprintf("every value of type %s is >= 0", basic))
			}
		} else {
			if (expr.Op == token.LSS || expr.Op == token.LEQ) && isobj(expr.Y, min) ||
				(expr.Op == token.GTR || expr.Op == token.GEQ) && isobj(expr.X, min) {
				report.Report(pass, expr, fmt.Sprintf("no value of type %s is less than %s", basic, min))
			}
			if expr.Op == token.GEQ && isobj(expr.Y, min) ||
				expr.Op == token.LEQ && isobj(expr.X, min) {
				report.Report(pass, expr, fmt.Sprintf("every value of type %s is >= %s", basic, min))
			}
		}

	}
	code.Preorder(pass, fn, (*ast.BinaryExpr)(nil))
	return nil, nil
}

func consts(val ir.Value, out []*ir.Const, visitedPhis map[string]bool) ([]*ir.Const, bool) {
	if visitedPhis == nil {
		visitedPhis = map[string]bool{}
	}
	var ok bool
	switch val := val.(type) {
	case *ir.Phi:
		if visitedPhis[val.Name()] {
			break
		}
		visitedPhis[val.Name()] = true
		vals := val.Operands(nil)
		for _, phival := range vals {
			out, ok = consts(*phival, out, visitedPhis)
			if !ok {
				return nil, false
			}
		}
	case *ir.Const:
		out = append(out, val)
	case *ir.Convert:
		out, ok = consts(val.X, out, visitedPhis)
		if !ok {
			return nil, false
		}
	default:
		return nil, false
	}
	if len(out) < 2 {
		return out, true
	}
	uniq := []*ir.Const{out[0]}
	for _, val := range out[1:] {
		if val.Value == uniq[len(uniq)-1].Value {
			continue
		}
		uniq = append(uniq, val)
	}
	return uniq, true
}

func CheckLoopCondition(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		cb := func(node ast.Node) bool {
			loop, ok := node.(*ast.ForStmt)
			if !ok {
				return true
			}
			if loop.Init == nil || loop.Cond == nil || loop.Post == nil {
				return true
			}
			init, ok := loop.Init.(*ast.AssignStmt)
			if !ok || len(init.Lhs) != 1 || len(init.Rhs) != 1 {
				return true
			}
			cond, ok := loop.Cond.(*ast.BinaryExpr)
			if !ok {
				return true
			}
			x, ok := cond.X.(*ast.Ident)
			if !ok {
				return true
			}
			lhs, ok := init.Lhs[0].(*ast.Ident)
			if !ok {
				return true
			}
			if x.Obj != lhs.Obj {
				return true
			}
			if _, ok := loop.Post.(*ast.IncDecStmt); !ok {
				return true
			}

			v, isAddr := fn.ValueForExpr(cond.X)
			if v == nil || isAddr {
				return true
			}
			switch v := v.(type) {
			case *ir.Phi:
				ops := v.Operands(nil)
				if len(ops) != 2 {
					return true
				}
				_, ok := (*ops[0]).(*ir.Const)
				if !ok {
					return true
				}
				sigma, ok := (*ops[1]).(*ir.Sigma)
				if !ok {
					return true
				}
				if sigma.X != v {
					return true
				}
			case *ir.Load:
				return true
			}
			report.Report(pass, cond, "variable in loop condition never changes")

			return true
		}
		if source := fn.Source(); source != nil {
			ast.Inspect(source, cb)
		}
	}
	return nil, nil
}

func CheckArgOverwritten(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		cb := func(node ast.Node) bool {
			var typ *ast.FuncType
			var body *ast.BlockStmt
			switch fn := node.(type) {
			case *ast.FuncDecl:
				typ = fn.Type
				body = fn.Body
			case *ast.FuncLit:
				typ = fn.Type
				body = fn.Body
			}
			if body == nil {
				return true
			}
			if len(typ.Params.List) == 0 {
				return true
			}
			for _, field := range typ.Params.List {
				for _, arg := range field.Names {
					obj := pass.TypesInfo.ObjectOf(arg)
					var irobj *ir.Parameter
					for _, param := range fn.Params {
						if param.Object() == obj {
							irobj = param
							break
						}
					}
					if irobj == nil {
						continue
					}
					refs := irobj.Referrers()
					if refs == nil {
						continue
					}
					if len(irutil.FilterDebug(*refs)) != 0 {
						continue
					}

					var assignment ast.Node
					ast.Inspect(body, func(node ast.Node) bool {
						if assignment != nil {
							return false
						}
						assign, ok := node.(*ast.AssignStmt)
						if !ok {
							return true
						}
						for _, lhs := range assign.Lhs {
							ident, ok := lhs.(*ast.Ident)
							if !ok {
								continue
							}
							if pass.TypesInfo.ObjectOf(ident) == obj {
								assignment = assign
								return false
							}
						}
						return true
					})
					if assignment != nil {
						report.Report(pass, arg, fmt.Sprintf("argument %s is overwritten before first use", arg),
							report.Related(assignment, fmt.Sprintf("assignment to %s", arg)))
					}
				}
			}
			return true
		}
		if source := fn.Source(); source != nil {
			ast.Inspect(source, cb)
		}
	}
	return nil, nil
}

func CheckIneffectiveLoop(pass *analysis.Pass) (interface{}, error) {
	// This check detects some, but not all unconditional loop exits.
	// We give up in the following cases:
	//
	// - a goto anywhere in the loop. The goto might skip over our
	// return, and we don't check that it doesn't.
	//
	// - any nested, unlabelled continue, even if it is in another
	// loop or closure.
	fn := func(node ast.Node) {
		var body *ast.BlockStmt
		switch fn := node.(type) {
		case *ast.FuncDecl:
			body = fn.Body
		case *ast.FuncLit:
			body = fn.Body
		default:
			lint.ExhaustiveTypeSwitch(node)
		}
		if body == nil {
			return
		}
		labels := map[*ast.Object]ast.Stmt{}
		ast.Inspect(body, func(node ast.Node) bool {
			label, ok := node.(*ast.LabeledStmt)
			if !ok {
				return true
			}
			labels[label.Label.Obj] = label.Stmt
			return true
		})

		ast.Inspect(body, func(node ast.Node) bool {
			var loop ast.Node
			var body *ast.BlockStmt
			switch node := node.(type) {
			case *ast.ForStmt:
				body = node.Body
				loop = node
			case *ast.RangeStmt:
				ok := typeutil.All(pass.TypesInfo.TypeOf(node.X), func(term *typeparams.Term) bool {
					switch term.Type().Underlying().(type) {
					case *types.Slice, *types.Chan, *types.Basic, *types.Pointer, *types.Array:
						return true
					case *types.Map:
						// looping once over a map is a valid pattern for
						// getting an arbitrary element.
						return false
					default:
						lint.ExhaustiveTypeSwitch(term.Type().Underlying())
						return false
					}
				})
				if !ok {
					return true
				}
				body = node.Body
				loop = node
			default:
				return true
			}
			if len(body.List) < 2 {
				// TODO(dh): is this check needed? when body.List < 2,
				// then we can't find both an unconditional exit and a
				// branching statement (if, ...). and we don't flag
				// unconditional exits if there has been no branching
				// in the loop body.

				// avoid flagging the somewhat common pattern of using
				// a range loop to get the first element in a slice,
				// or the first rune in a string.
				return true
			}
			var unconditionalExit ast.Node
			hasBranching := false
			for _, stmt := range body.List {
				switch stmt := stmt.(type) {
				case *ast.BranchStmt:
					switch stmt.Tok {
					case token.BREAK:
						if stmt.Label == nil || labels[stmt.Label.Obj] == loop {
							unconditionalExit = stmt
						}
					case token.CONTINUE:
						if stmt.Label == nil || labels[stmt.Label.Obj] == loop {
							unconditionalExit = nil
							return false
						}
					}
				case *ast.ReturnStmt:
					unconditionalExit = stmt
				case *ast.IfStmt, *ast.ForStmt, *ast.RangeStmt, *ast.SwitchStmt, *ast.SelectStmt:
					hasBranching = true
				}
			}
			if unconditionalExit == nil || !hasBranching {
				return false
			}
			ast.Inspect(body, func(node ast.Node) bool {
				if branch, ok := node.(*ast.BranchStmt); ok {

					switch branch.Tok {
					case token.GOTO:
						unconditionalExit = nil
						return false
					case token.CONTINUE:
						if branch.Label != nil && labels[branch.Label.Obj] != loop {
							return true
						}
						unconditionalExit = nil
						return false
					}
				}
				return true
			})
			if unconditionalExit != nil {
				report.Report(pass, unconditionalExit, "the surrounding loop is unconditionally terminated")
			}
			return true
		})
	}
	code.Preorder(pass, fn, (*ast.FuncDecl)(nil), (*ast.FuncLit)(nil))
	return nil, nil
}

var checkNilContextQ = pattern.MustParse(`(CallExpr fun@(Function _) (Builtin "nil"):_)`)

func CheckNilContext(pass *analysis.Pass) (interface{}, error) {
	todo := &ast.CallExpr{
		Fun: edit.Selector("context", "TODO"),
	}
	bg := &ast.CallExpr{
		Fun: edit.Selector("context", "Background"),
	}
	fn := func(node ast.Node) {
		m, ok := code.Match(pass, checkNilContextQ, node)
		if !ok {
			return
		}

		call := node.(*ast.CallExpr)
		fun, ok := m.State["fun"].(*types.Func)
		if !ok {
			// it might also be a builtin
			return
		}
		sig := fun.Type().(*types.Signature)
		if sig.Params().Len() == 0 {
			// Our CallExpr might've matched a method expression, like
			// (*T).Foo(nil) – here, nil isn't the first argument of
			// the Foo method, but the method receiver.
			return
		}
		if !typeutil.IsType(sig.Params().At(0).Type(), "context.Context") {
			return
		}
		report.Report(pass, call.Args[0],
			"do not pass a nil Context, even if a function permits it; pass context.TODO if you are unsure about which Context to use", report.Fixes(
				edit.Fix("use context.TODO", edit.ReplaceWithNode(pass.Fset, call.Args[0], todo)),
				edit.Fix("use context.Background", edit.ReplaceWithNode(pass.Fset, call.Args[0], bg))))
	}
	code.Preorder(pass, fn, (*ast.CallExpr)(nil))
	return nil, nil
}

var (
	checkSeekerQ = pattern.MustParse(`(CallExpr fun@(SelectorExpr _ (Ident "Seek")) [arg1@(SelectorExpr (Ident "io") (Ident (Or "SeekStart" "SeekCurrent" "SeekEnd"))) arg2])`)
	checkSeekerR = pattern.MustParse(`(CallExpr fun [arg2 arg1])`)
)

func CheckSeeker(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		if _, edits, ok := code.MatchAndEdit(pass, checkSeekerQ, checkSeekerR, node); ok {
			report.Report(pass, node, "the first argument of io.Seeker is the offset, but an io.Seek* constant is being used instead",
				report.Fixes(edit.Fix("swap arguments", edits...)))
		}
	}
	code.Preorder(pass, fn, (*ast.CallExpr)(nil))
	return nil, nil
}

func CheckIneffectiveAppend(pass *analysis.Pass) (interface{}, error) {
	isAppend := func(ins ir.Value) bool {
		call, ok := ins.(*ir.Call)
		if !ok {
			return false
		}
		if call.Call.IsInvoke() {
			return false
		}
		if builtin, ok := call.Call.Value.(*ir.Builtin); !ok || builtin.Name() != "append" {
			return false
		}
		return true
	}

	// We have to be careful about aliasing.
	// Multiple slices may refer to the same backing array,
	// making appends observable even when we don't see the result of append be used anywhere.
	//
	// We will have to restrict ourselves to slices that have been allocated within the function,
	// haven't been sliced,
	// and haven't been passed anywhere that could retain them (such as function calls or memory stores).
	//
	// We check whether an append should be flagged in two steps.
	//
	// In the first step, we look at the data flow graph, starting in reverse from the argument to append, till we reach the root.
	// This graph must only consist of the following instructions:
	//
	// - phi
	// - sigma
	// - slice
	// - const nil
	// - MakeSlice
	// - Alloc
	// - calls to append
	//
	// If this step succeeds, we look at all referrers of the values found in the first step, recursively.
	// These referrers must either be in the set of values found in the first step,
	// be DebugRefs,
	// or fulfill the same type requirements as step 1, with the exception of appends, which are forbidden.
	//
	// If both steps succeed then we know that the backing array hasn't been aliased in an observable manner.
	//
	// We could relax these restrictions by making use of additional information:
	// - if we passed the slice to a function that doesn't retain the slice then we can still flag it
	// - if a slice has been sliced but is dead afterwards, we can flag appends to the new slice

	// OPT(dh): We could cache the results of both validate functions.
	// However, we only use these functions on values that we otherwise want to flag, which are very few.
	// Not caching values hasn't increased the runtimes for the standard library nor k8s.
	var validateArgument func(v ir.Value, seen map[ir.Value]struct{}) bool
	validateArgument = func(v ir.Value, seen map[ir.Value]struct{}) bool {
		if _, ok := seen[v]; ok {
			// break cycle
			return true
		}
		seen[v] = struct{}{}
		switch v := v.(type) {
		case *ir.Phi:
			for _, edge := range v.Edges {
				if !validateArgument(edge, seen) {
					return false
				}
			}
			return true
		case *ir.Sigma:
			return validateArgument(v.X, seen)
		case *ir.Slice:
			return validateArgument(v.X, seen)
		case *ir.Const:
			return true
		case *ir.MakeSlice:
			return true
		case *ir.Alloc:
			return true
		case *ir.Call:
			if isAppend(v) {
				return validateArgument(v.Call.Args[0], seen)
			}
			return false
		default:
			return false
		}
	}

	var validateReferrers func(v ir.Value, seen map[ir.Instruction]struct{}) bool
	validateReferrers = func(v ir.Value, seen map[ir.Instruction]struct{}) bool {
		for _, ref := range *v.Referrers() {
			if _, ok := seen[ref]; ok {
				continue
			}

			seen[ref] = struct{}{}
			switch ref.(type) {
			case *ir.Phi:
			case *ir.Sigma:
			case *ir.Slice:
			case *ir.Const:
			case *ir.MakeSlice:
			case *ir.Alloc:
			case *ir.DebugRef:
			default:
				return false
			}

			if ref, ok := ref.(ir.Value); ok {
				if !validateReferrers(ref, seen) {
					return false
				}
			}
		}
		return true
	}

	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, block := range fn.Blocks {
			for _, ins := range block.Instrs {
				val, ok := ins.(ir.Value)
				if !ok || !isAppend(val) {
					continue
				}

				isUsed := false
				visited := map[ir.Instruction]bool{}
				var walkRefs func(refs []ir.Instruction)
				walkRefs = func(refs []ir.Instruction) {
				loop:
					for _, ref := range refs {
						if visited[ref] {
							continue
						}
						visited[ref] = true
						if _, ok := ref.(*ir.DebugRef); ok {
							continue
						}
						switch ref := ref.(type) {
						case *ir.Phi:
							walkRefs(*ref.Referrers())
						case *ir.Sigma:
							walkRefs(*ref.Referrers())
						case ir.Value:
							if !isAppend(ref) {
								isUsed = true
							} else {
								walkRefs(*ref.Referrers())
							}
						case ir.Instruction:
							isUsed = true
							break loop
						}
					}
				}

				refs := val.Referrers()
				if refs == nil {
					continue
				}
				walkRefs(*refs)

				if isUsed {
					continue
				}

				seen := map[ir.Value]struct{}{}
				if !validateArgument(ins.(*ir.Call).Call.Args[0], seen) {
					continue
				}

				seen2 := map[ir.Instruction]struct{}{}
				for k := range seen {
					// the only values we allow are also instructions, so this type assertion cannot fail
					seen2[k.(ir.Instruction)] = struct{}{}
				}
				seen2[ins] = struct{}{}
				failed := false
				for v := range seen {
					if !validateReferrers(v, seen2) {
						failed = true
						break
					}
				}
				if !failed {
					report.Report(pass, ins, "this result of append is never used, except maybe in other appends")
				}
			}
		}
	}
	return nil, nil
}

func CheckConcurrentTesting(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, block := range fn.Blocks {
			for _, ins := range block.Instrs {
				gostmt, ok := ins.(*ir.Go)
				if !ok {
					continue
				}
				var fn *ir.Function
				switch val := gostmt.Call.Value.(type) {
				case *ir.Function:
					fn = val
				case *ir.MakeClosure:
					fn = val.Fn.(*ir.Function)
				default:
					continue
				}
				if fn.Blocks == nil {
					continue
				}
				for _, block := range fn.Blocks {
					for _, ins := range block.Instrs {
						call, ok := ins.(*ir.Call)
						if !ok {
							continue
						}
						if call.Call.IsInvoke() {
							continue
						}
						callee := call.Call.StaticCallee()
						if callee == nil {
							continue
						}
						recv := callee.Signature.Recv()
						if recv == nil {
							continue
						}
						if !typeutil.IsType(recv.Type(), "*testing.common") {
							continue
						}
						fn, ok := call.Call.StaticCallee().Object().(*types.Func)
						if !ok {
							continue
						}
						name := fn.Name()
						switch name {
						case "FailNow", "Fatal", "Fatalf", "SkipNow", "Skip", "Skipf":
						default:
							continue
						}
						// TODO(dh): don't report multiple diagnostics
						// for multiple calls to T.Fatal, but do
						// collect all of them as related information
						report.Report(pass, gostmt, fmt.Sprintf("the goroutine calls T.%s, which must be called in the same goroutine as the test", name),
							report.Related(call, fmt.Sprintf("call to T.%s", name)))
					}
				}
			}
		}
	}
	return nil, nil
}

func eachCall(fn *ir.Function, cb func(caller *ir.Function, site ir.CallInstruction, callee *ir.Function)) {
	for _, b := range fn.Blocks {
		for _, instr := range b.Instrs {
			if site, ok := instr.(ir.CallInstruction); ok {
				if g := site.Common().StaticCallee(); g != nil {
					cb(fn, site, g)
				}
			}
		}
	}
}

func CheckCyclicFinalizer(pass *analysis.Pass) (interface{}, error) {
	cb := func(caller *ir.Function, site ir.CallInstruction, callee *ir.Function) {
		if callee.RelString(nil) != "runtime.SetFinalizer" {
			return
		}
		arg0 := site.Common().Args[knowledge.Arg("runtime.SetFinalizer.obj")]
		if iface, ok := arg0.(*ir.MakeInterface); ok {
			arg0 = iface.X
		}
		load, ok := arg0.(*ir.Load)
		if !ok {
			return
		}
		v, ok := load.X.(*ir.Alloc)
		if !ok {
			return
		}
		arg1 := site.Common().Args[knowledge.Arg("runtime.SetFinalizer.finalizer")]
		if iface, ok := arg1.(*ir.MakeInterface); ok {
			arg1 = iface.X
		}
		mc, ok := arg1.(*ir.MakeClosure)
		if !ok {
			return
		}
		for _, b := range mc.Bindings {
			if b == v {
				pos := report.DisplayPosition(pass.Fset, mc.Fn.Pos())
				report.Report(pass, site, fmt.Sprintf("the finalizer closes over the object, preventing the finalizer from ever running (at %s)", pos))
			}
		}
	}
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		eachCall(fn, cb)
	}
	return nil, nil
}

/*
func CheckSliceOutOfBounds(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, block := range fn.Blocks {
			for _, ins := range block.Instrs {
				ia, ok := ins.(*ir.IndexAddr)
				if !ok {
					continue
				}
				if _, ok := ia.X.Type().Underlying().(*types.Slice); !ok {
					continue
				}
				sr, ok1 := c.funcDescs.Get(fn).Ranges[ia.X].(vrp.SliceInterval)
				idxr, ok2 := c.funcDescs.Get(fn).Ranges[ia.Index].(vrp.IntInterval)
				if !ok1 || !ok2 || !sr.IsKnown() || !idxr.IsKnown() || sr.Length.Empty() || idxr.Empty() {
					continue
				}
				if idxr.Lower.Cmp(sr.Length.Upper) >= 0 {
					report.Nodef(pass, ia, "index out of bounds")
				}
			}
		}
	}
	return nil, nil
}
*/

func CheckDeferLock(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, block := range fn.Blocks {
			instrs := irutil.FilterDebug(block.Instrs)
			if len(instrs) < 2 {
				continue
			}
			for i, ins := range instrs[:len(instrs)-1] {
				call, ok := ins.(*ir.Call)
				if !ok {
					continue
				}
				if !irutil.IsCallToAny(call.Common(), "(*sync.Mutex).Lock", "(*sync.RWMutex).RLock") {
					continue
				}
				nins, ok := instrs[i+1].(*ir.Defer)
				if !ok {
					continue
				}
				if !irutil.IsCallToAny(&nins.Call, "(*sync.Mutex).Lock", "(*sync.RWMutex).RLock") {
					continue
				}
				if call.Common().Args[0] != nins.Call.Args[0] {
					continue
				}
				name := shortCallName(call.Common())
				alt := ""
				switch name {
				case "Lock":
					alt = "Unlock"
				case "RLock":
					alt = "RUnlock"
				}
				report.Report(pass, nins, fmt.Sprintf("deferring %s right after having locked already; did you mean to defer %s?", name, alt))
			}
		}
	}
	return nil, nil
}

func CheckNaNComparison(pass *analysis.Pass) (interface{}, error) {
	isNaN := func(v ir.Value) bool {
		call, ok := v.(*ir.Call)
		if !ok {
			return false
		}
		return irutil.IsCallTo(call.Common(), "math.NaN")
	}
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, block := range fn.Blocks {
			for _, ins := range block.Instrs {
				ins, ok := ins.(*ir.BinOp)
				if !ok {
					continue
				}
				if isNaN(irutil.Flatten(ins.X)) || isNaN(irutil.Flatten(ins.Y)) {
					report.Report(pass, ins, "no value is equal to NaN, not even NaN itself")
				}
			}
		}
	}
	return nil, nil
}

func CheckInfiniteRecursion(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		eachCall(fn, func(caller *ir.Function, site ir.CallInstruction, callee *ir.Function) {
			if callee != fn {
				return
			}
			if _, ok := site.(*ir.Go); ok {
				// Recursively spawning goroutines doesn't consume
				// stack space infinitely, so don't flag it.
				return
			}

			block := site.Block()
			for _, b := range fn.Blocks {
				if block.Dominates(b) {
					continue
				}
				if len(b.Instrs) == 0 {
					continue
				}
				if _, ok := b.Control().(*ir.Return); ok {
					return
				}
			}
			report.Report(pass, site, "infinite recursive call")
		})
	}
	return nil, nil
}

func CheckLeakyTimeTick(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		if code.IsMainLike(pass) || code.IsInTest(pass, fn) {
			continue
		}
		for _, block := range fn.Blocks {
			for _, ins := range block.Instrs {
				call, ok := ins.(*ir.Call)
				if !ok || !irutil.IsCallTo(call.Common(), "time.Tick") {
					continue
				}
				if !irutil.Terminates(call.Parent()) {
					continue
				}
				report.Report(pass, call, "using time.Tick leaks the underlying ticker, consider using it only in endless functions, tests and the main package, and use time.NewTicker here")
			}
		}
	}
	return nil, nil
}

var checkDoubleNegationQ = pattern.MustParse(`(UnaryExpr "!" single@(UnaryExpr "!" x))`)

func CheckDoubleNegation(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		if m, ok := code.Match(pass, checkDoubleNegationQ, node); ok {
			report.Report(pass, node, "negating a boolean twice has no effect; is this a typo?", report.Fixes(
				edit.Fix("turn into single negation", edit.ReplaceWithNode(pass.Fset, node, m.State["single"].(ast.Node))),
				edit.Fix("remove double negation", edit.ReplaceWithNode(pass.Fset, node, m.State["x"].(ast.Node)))))
		}
	}
	code.Preorder(pass, fn, (*ast.UnaryExpr)(nil))
	return nil, nil
}

func CheckRepeatedIfElse(pass *analysis.Pass) (interface{}, error) {
	seen := map[ast.Node]bool{}

	var collectConds func(ifstmt *ast.IfStmt, conds []ast.Expr) ([]ast.Expr, bool)
	collectConds = func(ifstmt *ast.IfStmt, conds []ast.Expr) ([]ast.Expr, bool) {
		seen[ifstmt] = true
		// Bail if any if-statement has an Init statement or side effects in its condition
		if ifstmt.Init != nil {
			return nil, false
		}
		if code.MayHaveSideEffects(pass, ifstmt.Cond, nil) {
			return nil, false
		}

		conds = append(conds, ifstmt.Cond)
		if elsestmt, ok := ifstmt.Else.(*ast.IfStmt); ok {
			return collectConds(elsestmt, conds)
		}
		return conds, true
	}
	fn := func(node ast.Node) {
		ifstmt := node.(*ast.IfStmt)
		if seen[ifstmt] {
			// this if-statement is part of an if/else-if chain that we've already processed
			return
		}
		if ifstmt.Else == nil {
			// there can be at most one condition
			return
		}
		conds, ok := collectConds(ifstmt, nil)
		if !ok {
			return
		}
		if len(conds) < 2 {
			return
		}
		counts := map[string]int{}
		for _, cond := range conds {
			s := report.Render(pass, cond)
			counts[s]++
			if counts[s] == 2 {
				report.Report(pass, cond, "this condition occurs multiple times in this if/else if chain")
			}
		}
	}
	code.Preorder(pass, fn, (*ast.IfStmt)(nil))
	return nil, nil
}

func CheckSillyBitwiseOps(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		binop := node.(*ast.BinaryExpr)
		if !typeutil.All(pass.TypesInfo.TypeOf(binop), func(term *typeparams.Term) bool {
			b, ok := term.Type().Underlying().(*types.Basic)
			if !ok {
				return false
			}
			return (b.Info() & types.IsInteger) != 0
		}) {
			return
		}
		switch binop.Op {
		case token.AND, token.OR, token.XOR:
		default:
			// we do not flag shifts because too often, x<<0 is part
			// of a pattern, x<<0, x<<8, x<<16, ...
			return
		}
		if y, ok := binop.Y.(*ast.Ident); ok {
			obj, ok := pass.TypesInfo.ObjectOf(y).(*types.Const)
			if !ok {
				return
			}
			if obj.Pkg() != pass.Pkg {
				// identifier was dot-imported
				return
			}
			if v, _ := constant.Int64Val(obj.Val()); v != 0 {
				return
			}
			path, _ := astutil.PathEnclosingInterval(code.File(pass, obj), obj.Pos(), obj.Pos())
			if len(path) < 2 {
				return
			}
			spec, ok := path[1].(*ast.ValueSpec)
			if !ok {
				return
			}
			if len(spec.Names) != 1 || len(spec.Values) != 1 {
				// TODO(dh): we could support this
				return
			}
			ident, ok := spec.Values[0].(*ast.Ident)
			if !ok {
				return
			}
			if !isIota(pass.TypesInfo.ObjectOf(ident)) {
				return
			}
			switch binop.Op {
			case token.AND:
				report.Report(pass, node,
					fmt.Sprintf("%s always equals 0; %s is defined as iota and has value 0, maybe %s is meant to be 1 << iota?", report.Render(pass, binop), report.Render(pass, binop.Y), report.Render(pass, binop.Y)))
			case token.OR, token.XOR:
				report.Report(pass, node,
					fmt.Sprintf("%s always equals %s; %s is defined as iota and has value 0, maybe %s is meant to be 1 << iota?", report.Render(pass, binop), report.Render(pass, binop.X), report.Render(pass, binop.Y), report.Render(pass, binop.Y)))
			}
		} else if code.IsIntegerLiteral(pass, binop.Y, constant.MakeInt64(0)) {
			switch binop.Op {
			case token.AND:
				report.Report(pass, node, fmt.Sprintf("%s always equals 0", report.Render(pass, binop)))
			case token.OR, token.XOR:
				report.Report(pass, node, fmt.Sprintf("%s always equals %s", report.Render(pass, binop), report.Render(pass, binop.X)))
			}
		}
	}
	code.Preorder(pass, fn, (*ast.BinaryExpr)(nil))
	return nil, nil
}

func isIota(obj types.Object) bool {
	if obj.Name() != "iota" {
		return false
	}
	c, ok := obj.(*types.Const)
	if !ok {
		return false
	}
	return c.Pkg() == nil
}

func CheckNonOctalFileMode(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		call := node.(*ast.CallExpr)
		for _, arg := range call.Args {
			lit, ok := arg.(*ast.BasicLit)
			if !ok {
				continue
			}
			if !typeutil.IsType(pass.TypesInfo.TypeOf(lit), "os.FileMode") &&
				!typeutil.IsType(pass.TypesInfo.TypeOf(lit), "io/fs.FileMode") {
				continue
			}
			if len(lit.Value) == 3 &&
				lit.Value[0] != '0' &&
				lit.Value[0] >= '0' && lit.Value[0] <= '7' &&
				lit.Value[1] >= '0' && lit.Value[1] <= '7' &&
				lit.Value[2] >= '0' && lit.Value[2] <= '7' {

				v, err := strconv.ParseInt(lit.Value, 10, 64)
				if err != nil {
					continue
				}
				report.Report(pass, arg, fmt.Sprintf("file mode '%s' evaluates to %#o; did you mean '0%s'?", lit.Value, v, lit.Value),
					report.Fixes(edit.Fix("fix octal literal", edit.ReplaceWithString(arg, "0"+lit.Value))))
			}
		}
	}
	code.Preorder(pass, fn, (*ast.CallExpr)(nil))
	return nil, nil
}

func CheckPureFunctions(pass *analysis.Pass) (interface{}, error) {
	pure := pass.ResultOf[facts.Purity].(facts.PurityResult)

fnLoop:
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		if code.IsInTest(pass, fn) {
			params := fn.Signature.Params()
			for i := 0; i < params.Len(); i++ {
				param := params.At(i)
				if typeutil.IsType(param.Type(), "*testing.B") {
					// Ignore discarded pure functions in code related
					// to benchmarks. Instead of matching BenchmarkFoo
					// functions, we match any function accepting a
					// *testing.B. Benchmarks sometimes call generic
					// functions for doing the actual work, and
					// checking for the parameter is a lot easier and
					// faster than analyzing call trees.
					continue fnLoop
				}
			}
		}

		for _, b := range fn.Blocks {
			for _, ins := range b.Instrs {
				ins, ok := ins.(*ir.Call)
				if !ok {
					continue
				}
				refs := ins.Referrers()
				if refs == nil || len(irutil.FilterDebug(*refs)) > 0 {
					continue
				}

				callee := ins.Common().StaticCallee()
				if callee == nil {
					continue
				}
				if callee.Object() == nil {
					// TODO(dh): support anonymous functions
					continue
				}
				if _, ok := pure[callee.Object().(*types.Func)]; ok {
					if pass.Pkg.Path() == "fmt_test" && callee.Object().(*types.Func).FullName() == "fmt.Sprintf" {
						// special case for benchmarks in the fmt package
						continue
					}
					report.Report(pass, ins, fmt.Sprintf("%s is a pure function but its return value is ignored", callee.Object().Name()))
				}
			}
		}
	}
	return nil, nil
}

func CheckDeprecated(pass *analysis.Pass) (interface{}, error) {
	deprs := pass.ResultOf[facts.Deprecated].(facts.DeprecatedResult)

	// Selectors can appear outside of function literals, e.g. when
	// declaring package level variables.

	isStdlib := func(obj types.Object) bool {
		// Modules with no dot in the first path element are reserved for the standard library and tooling.
		// This is the best we can currently do.
		// Nobody tells us which import paths are part of the standard library.
		//
		// We check the entire path instead of just the first path element, because the standard library doesn't contain paths with any dots, anyway.

		return !strings.Contains(obj.Pkg().Path(), ".")
	}

	var tfn types.Object
	stack := 0
	fn := func(node ast.Node, push bool) bool {
		if !push {
			stack--
			return false
		}
		stack++
		if stack == 1 {
			tfn = nil
		}
		if fn, ok := node.(*ast.FuncDecl); ok {
			tfn = pass.TypesInfo.ObjectOf(fn.Name)
		}
		sel, ok := node.(*ast.SelectorExpr)
		if !ok {
			return true
		}

		obj := pass.TypesInfo.ObjectOf(sel.Sel)
		if obj_, ok := obj.(*types.Func); ok {
			obj = typeparams.OriginMethod(obj_)
		}
		if obj.Pkg() == nil {
			return true
		}
		if pass.Pkg == obj.Pkg() || obj.Pkg().Path()+"_test" == pass.Pkg.Path() {
			// Don't flag stuff in our own package
			return true
		}

		if depr, ok := deprs.Objects[obj]; ok {
			// Note: gopls doesn't correctly run analyzers on
			// dependencies, so we'll never be able to find deprecated
			// objects in imported code. We've experimented with
			// lifting the stdlib handling out of the general check,
			// to at least work for deprecated objects in the stdlib,
			// but we gave up on that, because we wouldn't have access
			// to the deprecation message.
			std, ok := knowledge.StdlibDeprecations[code.SelectorName(pass, sel)]
			if !ok && isStdlib(obj) {
				// Deprecated object in the standard library, but we don't know the details of the deprecation.
				// Don't flag it at all, to avoid flagging an object that was deprecated in 1.N when targeting 1.N-1.
				// See https://staticcheck.io/issues/1108 for the background on this.
				return true
			}
			if ok {
				switch std.AlternativeAvailableSince {
				case knowledge.DeprecatedNeverUse:
					// This should never be used, regardless of the
					// targeted Go version. Examples include insecure
					// cryptography or inherently broken APIs.
					//
					// We always want to flag these.
				case knowledge.DeprecatedUseNoLonger:
					// This should no longer be used. Using it with
					// older Go versions might still make sense.
					if !code.IsGoVersion(pass, std.DeprecatedSince) {
						return true
					}
				default:
					if std.AlternativeAvailableSince < 0 {
						panic(fmt.Sprintf("unhandled case %d", std.AlternativeAvailableSince))
					}
					// Look for the first available alternative, not the first
					// version something was deprecated in. If a function was
					// deprecated in Go 1.6, an alternative has been available
					// already in 1.0, and we're targeting 1.2, it still
					// makes sense to use the alternative from 1.0, to be
					// future-proof.
					if !code.IsGoVersion(pass, std.AlternativeAvailableSince) {
						return true
					}
				}
			}

			if tfn != nil {
				if _, ok := deprs.Objects[tfn]; ok {
					// functions that are deprecated may use deprecated
					// symbols
					return true
				}
			}

			if ok {
				if std.AlternativeAvailableSince == knowledge.DeprecatedNeverUse {
					report.Report(pass, sel, fmt.Sprintf("%s has been deprecated since Go 1.%d because it shouldn't be used: %s", report.Render(pass, sel), std.DeprecatedSince, depr.Msg))
				} else if std.AlternativeAvailableSince == std.DeprecatedSince || std.AlternativeAvailableSince == knowledge.DeprecatedUseNoLonger {
					report.Report(pass, sel, fmt.Sprintf("%s has been deprecated since Go 1.%d: %s", report.Render(pass, sel), std.DeprecatedSince, depr.Msg))
				} else {
					report.Report(pass, sel, fmt.Sprintf("%s has been deprecated since Go 1.%d and an alternative has been available since Go 1.%d: %s", report.Render(pass, sel), std.DeprecatedSince, std.AlternativeAvailableSince, depr.Msg))
				}
			} else {
				report.Report(pass, sel, fmt.Sprintf("%s is deprecated: %s", report.Render(pass, sel), depr.Msg))
			}
			return true
		}
		return true
	}

	fn2 := func(node ast.Node) {
		spec := node.(*ast.ImportSpec)
		var imp *types.Package
		if spec.Name != nil {
			imp = pass.TypesInfo.ObjectOf(spec.Name).(*types.PkgName).Imported()
		} else {
			imp = pass.TypesInfo.Implicits[spec].(*types.PkgName).Imported()
		}

		p := spec.Path.Value
		path := p[1 : len(p)-1]
		if depr, ok := deprs.Packages[imp]; ok {
			if path == "github.com/golang/protobuf/proto" {
				gen, ok := code.Generator(pass, spec.Path.Pos())
				if ok && gen == facts.ProtocGenGo {
					return
				}
			}
			report.Report(pass, spec, fmt.Sprintf("package %s is deprecated: %s", path, depr.Msg))
		}
	}
	pass.ResultOf[inspect.Analyzer].(*inspector.Inspector).Nodes(nil, fn)
	code.Preorder(pass, fn2, (*ast.ImportSpec)(nil))
	return nil, nil
}

func callChecker(rules map[string]CallCheck) func(pass *analysis.Pass) (interface{}, error) {
	return func(pass *analysis.Pass) (interface{}, error) {
		return checkCalls(pass, rules)
	}
}

func checkCalls(pass *analysis.Pass, rules map[string]CallCheck) (interface{}, error) {
	cb := func(caller *ir.Function, site ir.CallInstruction, callee *ir.Function) {
		obj, ok := callee.Object().(*types.Func)
		if !ok {
			return
		}

		r, ok := rules[typeutil.FuncName(obj)]
		if !ok {
			return
		}
		var args []*Argument
		irargs := site.Common().Args
		if callee.Signature.Recv() != nil {
			irargs = irargs[1:]
		}
		for _, arg := range irargs {
			if iarg, ok := arg.(*ir.MakeInterface); ok {
				arg = iarg.X
			}
			args = append(args, &Argument{Value: Value{arg}})
		}
		call := &Call{
			Pass:   pass,
			Instr:  site,
			Args:   args,
			Parent: site.Parent(),
		}
		r(call)

		var astcall *ast.CallExpr
		switch source := site.Source().(type) {
		case *ast.CallExpr:
			astcall = source
		case *ast.DeferStmt:
			astcall = source.Call
		case *ast.GoStmt:
			astcall = source.Call
		case nil:
			// TODO(dh): I am not sure this can actually happen. If it
			// can't, we should remove this case, and also stop
			// checking for astcall == nil in the code that follows.
		default:
			panic(fmt.Sprintf("unhandled case %T", source))
		}

		for idx, arg := range call.Args {
			for _, e := range arg.invalids {
				if astcall != nil {
					if idx < len(astcall.Args) {
						report.Report(pass, astcall.Args[idx], e)
					} else {
						// this is an instance of fn1(fn2()) where fn2
						// returns multiple values. Report the error
						// at the next-best position that we have, the
						// first argument. An example of a check that
						// triggers this is checkEncodingBinaryRules.
						report.Report(pass, astcall.Args[0], e)
					}
				} else {
					report.Report(pass, site, e)
				}
			}
		}
		for _, e := range call.invalids {
			report.Report(pass, call.Instr, e)
		}
	}
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		eachCall(fn, cb)
	}
	return nil, nil
}

func shortCallName(call *ir.CallCommon) string {
	if call.IsInvoke() {
		return ""
	}
	switch v := call.Value.(type) {
	case *ir.Function:
		fn, ok := v.Object().(*types.Func)
		if !ok {
			return ""
		}
		return fn.Name()
	case *ir.Builtin:
		return v.Name()
	}
	return ""
}

func CheckWriterBufferModified(pass *analysis.Pass) (interface{}, error) {
	// TODO(dh): this might be a good candidate for taint analysis.
	// Taint the argument as MUST_NOT_MODIFY, then propagate that
	// through functions like bytes.Split

	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		sig := fn.Signature
		if fn.Name() != "Write" || sig.Recv() == nil || sig.Params().Len() != 1 || sig.Results().Len() != 2 {
			continue
		}
		tArg, ok := sig.Params().At(0).Type().(*types.Slice)
		if !ok {
			continue
		}
		if basic, ok := tArg.Elem().(*types.Basic); !ok || basic.Kind() != types.Byte {
			continue
		}
		if basic, ok := sig.Results().At(0).Type().(*types.Basic); !ok || basic.Kind() != types.Int {
			continue
		}
		if named, ok := sig.Results().At(1).Type().(*types.Named); !ok || !typeutil.IsType(named, "error") {
			continue
		}

		for _, block := range fn.Blocks {
			for _, ins := range block.Instrs {
				switch ins := ins.(type) {
				case *ir.Store:
					addr, ok := ins.Addr.(*ir.IndexAddr)
					if !ok {
						continue
					}
					if addr.X != fn.Params[1] {
						continue
					}
					report.Report(pass, ins, "io.Writer.Write must not modify the provided buffer, not even temporarily")
				case *ir.Call:
					if !irutil.IsCallTo(ins.Common(), "append") {
						continue
					}
					if ins.Common().Args[0] != fn.Params[1] {
						continue
					}
					report.Report(pass, ins, "io.Writer.Write must not modify the provided buffer, not even temporarily")
				}
			}
		}
	}
	return nil, nil
}

func loopedRegexp(name string) CallCheck {
	return func(call *Call) {
		if extractConst(call.Args[0].Value.Value) == nil {
			return
		}
		if !isInLoop(call.Instr.Block()) {
			return
		}
		call.Invalid(fmt.Sprintf("calling %s in a loop has poor performance, consider using regexp.Compile", name))
	}
}

func CheckEmptyBranch(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		if fn.Source() == nil {
			continue
		}
		if irutil.IsExample(fn) {
			continue
		}
		cb := func(node ast.Node) bool {
			ifstmt, ok := node.(*ast.IfStmt)
			if !ok {
				return true
			}
			if ifstmt.Else != nil {
				b, ok := ifstmt.Else.(*ast.BlockStmt)
				if !ok || len(b.List) != 0 {
					return true
				}
				report.Report(pass, ifstmt.Else, "empty branch", report.FilterGenerated(), report.ShortRange())
			}
			if len(ifstmt.Body.List) != 0 {
				return true
			}
			report.Report(pass, ifstmt, "empty branch", report.FilterGenerated(), report.ShortRange())
			return true
		}
		if source := fn.Source(); source != nil {
			ast.Inspect(source, cb)
		}
	}
	return nil, nil
}

func CheckMapBytesKey(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, b := range fn.Blocks {
		insLoop:
			for _, ins := range b.Instrs {
				// find []byte -> string conversions
				conv, ok := ins.(*ir.Convert)
				if !ok || conv.Type() != types.Universe.Lookup("string").Type() {
					continue
				}
				tset := typeutil.NewTypeSet(conv.X.Type())
				// If at least one of the types is []byte, then it's more efficient to inline the conversion
				if !tset.Any(func(term *typeparams.Term) bool {
					s, ok := term.Type().Underlying().(*types.Slice)
					return ok && s.Elem().Underlying() == types.Universe.Lookup("byte").Type()
				}) {
					continue
				}
				refs := conv.Referrers()
				// need at least two (DebugRef) references: the
				// conversion and the *ast.Ident
				if refs == nil || len(*refs) < 2 {
					continue
				}
				ident := false
				// skip first reference, that's the conversion itself
				for _, ref := range (*refs)[1:] {
					switch ref := ref.(type) {
					case *ir.DebugRef:
						if _, ok := ref.Expr.(*ast.Ident); !ok {
							// the string seems to be used somewhere
							// unexpected; the default branch should
							// catch this already, but be safe
							continue insLoop
						} else {
							ident = true
						}
					case *ir.MapLookup:
					default:
						// the string is used somewhere else than a
						// map lookup
						continue insLoop
					}
				}

				// the result of the conversion wasn't assigned to an
				// identifier
				if !ident {
					continue
				}
				report.Report(pass, conv, "m[string(key)] would be more efficient than k := string(key); m[k]")
			}
		}
	}
	return nil, nil
}

func CheckRangeStringRunes(pass *analysis.Pass) (interface{}, error) {
	return sharedcheck.CheckRangeStringRunes(pass)
}

func CheckSelfAssignment(pass *analysis.Pass) (interface{}, error) {
	pure := pass.ResultOf[facts.Purity].(facts.PurityResult)

	fn := func(node ast.Node) {
		assign := node.(*ast.AssignStmt)
		if assign.Tok != token.ASSIGN || len(assign.Lhs) != len(assign.Rhs) {
			return
		}
		for i, lhs := range assign.Lhs {
			rhs := assign.Rhs[i]
			if reflect.TypeOf(lhs) != reflect.TypeOf(rhs) {
				continue
			}
			if code.MayHaveSideEffects(pass, lhs, pure) || code.MayHaveSideEffects(pass, rhs, pure) {
				continue
			}

			rlh := report.Render(pass, lhs)
			rrh := report.Render(pass, rhs)
			if rlh == rrh {
				report.Report(pass, assign, fmt.Sprintf("self-assignment of %s to %s", rrh, rlh), report.FilterGenerated())
			}
		}
	}
	code.Preorder(pass, fn, (*ast.AssignStmt)(nil))
	return nil, nil
}

func buildTagsIdentical(s1, s2 []string) bool {
	if len(s1) != len(s2) {
		return false
	}
	s1s := make([]string, len(s1))
	copy(s1s, s1)
	sort.Strings(s1s)
	s2s := make([]string, len(s2))
	copy(s2s, s2)
	sort.Strings(s2s)
	for i, s := range s1s {
		if s != s2s[i] {
			return false
		}
	}
	return true
}

func CheckDuplicateBuildConstraints(pass *analysis.Pass) (interface{}, error) {
	for _, f := range pass.Files {
		constraints := buildTags(f)
		for i, constraint1 := range constraints {
			for j, constraint2 := range constraints {
				if i >= j {
					continue
				}
				if buildTagsIdentical(constraint1, constraint2) {
					msg := fmt.Sprintf("identical build constraints %q and %q",
						strings.Join(constraint1, " "),
						strings.Join(constraint2, " "))
					report.Report(pass, f, msg, report.FilterGenerated(), report.ShortRange())
				}
			}
		}
	}
	return nil, nil
}

func CheckSillyRegexp(pass *analysis.Pass) (interface{}, error) {
	// We could use the rule checking engine for this, but the
	// arguments aren't really invalid.
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, b := range fn.Blocks {
			for _, ins := range b.Instrs {
				call, ok := ins.(*ir.Call)
				if !ok {
					continue
				}
				if !irutil.IsCallToAny(call.Common(), "regexp.MustCompile", "regexp.Compile", "regexp.Match", "regexp.MatchReader", "regexp.MatchString") {
					continue
				}
				c, ok := call.Common().Args[0].(*ir.Const)
				if !ok {
					continue
				}
				s := constant.StringVal(c.Value)
				re, err := syntax.Parse(s, 0)
				if err != nil {
					continue
				}
				if re.Op != syntax.OpLiteral && re.Op != syntax.OpEmptyMatch {
					continue
				}
				report.Report(pass, call, "regular expression does not contain any meta characters")
			}
		}
	}
	return nil, nil
}

func CheckMissingEnumTypesInDeclaration(pass *analysis.Pass) (interface{}, error) {
	convertibleTo := func(V, T types.Type) bool {
		if types.ConvertibleTo(V, T) {
			return true
		}
		// Go <1.16 returns false for untyped string to string conversion
		if V, ok := V.(*types.Basic); ok && V.Kind() == types.UntypedString {
			if T, ok := T.Underlying().(*types.Basic); ok && T.Kind() == types.String {
				return true
			}
		}
		return false
	}
	fn := func(node ast.Node) {
		decl := node.(*ast.GenDecl)
		if !decl.Lparen.IsValid() {
			return
		}
		if decl.Tok != token.CONST {
			return
		}

		groups := astutil.GroupSpecs(pass.Fset, decl.Specs)
	groupLoop:
		for _, group := range groups {
			if len(group) < 2 {
				continue
			}
			if group[0].(*ast.ValueSpec).Type == nil {
				// first constant doesn't have a type
				continue groupLoop
			}

			firstType := pass.TypesInfo.TypeOf(group[0].(*ast.ValueSpec).Values[0])
			for i, spec := range group {
				spec := spec.(*ast.ValueSpec)
				if i > 0 && spec.Type != nil {
					continue groupLoop
				}
				if len(spec.Names) != 1 || len(spec.Values) != 1 {
					continue groupLoop
				}

				if !convertibleTo(pass.TypesInfo.TypeOf(spec.Values[0]), firstType) {
					continue groupLoop
				}

				switch v := spec.Values[0].(type) {
				case *ast.BasicLit:
				case *ast.UnaryExpr:
					if _, ok := v.X.(*ast.BasicLit); !ok {
						continue groupLoop
					}
				default:
					// if it's not a literal it might be typed, such as
					// time.Microsecond = 1000 * Nanosecond
					continue groupLoop
				}
			}
			var edits []analysis.TextEdit
			typ := group[0].(*ast.ValueSpec).Type
			for _, spec := range group[1:] {
				nspec := *spec.(*ast.ValueSpec)
				nspec.Type = typ
				// The position of `spec` node excludes comments (if any).
				// However, on generating the source back from the node, the comments are included. Setting `Comment` to nil ensures deduplication of comments.
				nspec.Comment = nil
				edits = append(edits, edit.ReplaceWithNode(pass.Fset, spec, &nspec))
			}
			report.Report(pass, group[0], "only the first constant in this group has an explicit type", report.Fixes(edit.Fix("add type to all constants in group", edits...)))
		}
	}
	code.Preorder(pass, fn, (*ast.GenDecl)(nil))
	return nil, nil
}

func CheckTimerResetReturnValue(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, block := range fn.Blocks {
			for _, ins := range block.Instrs {
				call, ok := ins.(*ir.Call)
				if !ok {
					continue
				}
				if !irutil.IsCallTo(call.Common(), "(*time.Timer).Reset") {
					continue
				}
				refs := call.Referrers()
				if refs == nil {
					continue
				}
				for _, ref := range irutil.FilterDebug(*refs) {
					ifstmt, ok := ref.(*ir.If)
					if !ok {
						continue
					}

					found := false
					for _, succ := range ifstmt.Block().Succs {
						if len(succ.Preds) != 1 {
							// Merge point, not a branch in the
							// syntactical sense.

							// FIXME(dh): this is broken for if
							// statements a la "if x || y"
							continue
						}
						irutil.Walk(succ, func(b *ir.BasicBlock) bool {
							if !succ.Dominates(b) {
								// We've reached the end of the branch
								return false
							}
							for _, ins := range b.Instrs {
								// TODO(dh): we should check that
								// we're receiving from the channel of
								// a time.Timer to further reduce
								// false positives. Not a key
								// priority, considering the rarity of
								// Reset and the tiny likeliness of a
								// false positive
								if ins, ok := ins.(*ir.Recv); ok && typeutil.IsType(ins.Chan.Type(), "<-chan time.Time") {
									found = true
									return false
								}
							}
							return true
						})
					}

					if found {
						report.Report(pass, call, "it is not possible to use Reset's return value correctly, as there is a race condition between draining the channel and the new timer expiring")
					}
				}
			}
		}
	}
	return nil, nil
}

var (
	checkToLowerToUpperComparisonQ = pattern.MustParse(`
	(BinaryExpr
		(CallExpr fun@(Function (Or "strings.ToLower" "strings.ToUpper")) [a])
 		tok@(Or "==" "!=")
 		(CallExpr fun [b]))`)
	checkToLowerToUpperComparisonR = pattern.MustParse(`(CallExpr (SelectorExpr (Ident "strings") (Ident "EqualFold")) [a b])`)
)

func CheckToLowerToUpperComparison(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		m, ok := code.Match(pass, checkToLowerToUpperComparisonQ, node)
		if !ok {
			return
		}
		rn := pattern.NodeToAST(checkToLowerToUpperComparisonR.Root, m.State).(ast.Expr)
		if m.State["tok"].(token.Token) == token.NEQ {
			rn = &ast.UnaryExpr{
				Op: token.NOT,
				X:  rn,
			}
		}

		report.Report(pass, node, "should use strings.EqualFold instead", report.Fixes(edit.Fix("replace with strings.EqualFold", edit.ReplaceWithNode(pass.Fset, node, rn))))
	}

	code.Preorder(pass, fn, (*ast.BinaryExpr)(nil))
	return nil, nil
}

func CheckUnreachableTypeCases(pass *analysis.Pass) (interface{}, error) {
	// Check if T subsumes V in a type switch. T subsumes V if T is an interface and T's method set is a subset of V's method set.
	subsumes := func(T, V types.Type) bool {
		if typeparams.IsTypeParam(T) {
			return false
		}
		tIface, ok := T.Underlying().(*types.Interface)
		if !ok {
			return false
		}

		return types.Implements(V, tIface)
	}

	subsumesAny := func(Ts, Vs []types.Type) (types.Type, types.Type, bool) {
		for _, T := range Ts {
			for _, V := range Vs {
				if subsumes(T, V) {
					return T, V, true
				}
			}
		}

		return nil, nil, false
	}

	fn := func(node ast.Node) {
		tsStmt := node.(*ast.TypeSwitchStmt)

		type ccAndTypes struct {
			cc    *ast.CaseClause
			types []types.Type
		}

		// All asserted types in the order of case clauses.
		ccs := make([]ccAndTypes, 0, len(tsStmt.Body.List))
		for _, stmt := range tsStmt.Body.List {
			cc, _ := stmt.(*ast.CaseClause)

			// Exclude the 'default' case.
			if len(cc.List) == 0 {
				continue
			}

			Ts := make([]types.Type, 0, len(cc.List))
			for _, expr := range cc.List {
				// Exclude the 'nil' value from any 'case' statement (it is always reachable).
				if typ := pass.TypesInfo.TypeOf(expr); typ != types.Typ[types.UntypedNil] {
					Ts = append(Ts, typ)
				}
			}

			ccs = append(ccs, ccAndTypes{cc: cc, types: Ts})
		}

		if len(ccs) <= 1 {
			// Zero or one case clauses, nothing to check.
			return
		}

		// Check if case clauses following cc have types that are subsumed by cc.
		for i, cc := range ccs[:len(ccs)-1] {
			for _, next := range ccs[i+1:] {
				if T, V, yes := subsumesAny(cc.types, next.types); yes {
					report.Report(pass, next.cc, fmt.Sprintf("unreachable case clause: %s will always match before %s", T.String(), V.String()),
						report.ShortRange())
				}
			}
		}
	}

	code.Preorder(pass, fn, (*ast.TypeSwitchStmt)(nil))
	return nil, nil
}

var checkSingleArgAppendQ = pattern.MustParse(`(CallExpr (Builtin "append") [_])`)

func CheckSingleArgAppend(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		_, ok := code.Match(pass, checkSingleArgAppendQ, node)
		if !ok {
			return
		}
		report.Report(pass, node, "x = append(y) is equivalent to x = y", report.FilterGenerated())
	}
	code.Preorder(pass, fn, (*ast.CallExpr)(nil))
	return nil, nil
}

func CheckStructTags(pass *analysis.Pass) (interface{}, error) {
	importsGoFlags := false

	// we use the AST instead of (*types.Package).Imports to work
	// around vendored packages in GOPATH mode. A vendored package's
	// path will include the vendoring subtree as a prefix.
	for _, f := range pass.Files {
		for _, imp := range f.Imports {
			v := imp.Path.Value
			if v[1:len(v)-1] == "github.com/jessevdk/go-flags" {
				importsGoFlags = true
				break
			}
		}
	}

	fn := func(node ast.Node) {
		structNode := node.(*ast.StructType)
		T := pass.TypesInfo.Types[structNode].Type.(*types.Struct)
		rt := fakereflect.TypeAndCanAddr{
			Type: T,
		}
		for i, field := range structNode.Fields.List {
			if field.Tag == nil {
				continue
			}
			tags, err := parseStructTag(field.Tag.Value[1 : len(field.Tag.Value)-1])
			if err != nil {
				report.Report(pass, field.Tag, fmt.Sprintf("unparseable struct tag: %s", err))
				continue
			}
			for k, v := range tags {
				if len(v) > 1 {
					isGoFlagsTag := importsGoFlags &&
						(k == "choice" || k == "optional-value" || k == "default")
					if !isGoFlagsTag {
						report.Report(pass, field.Tag, fmt.Sprintf("duplicate struct tag %q", k))
					}
				}

				switch k {
				case "json":
					checkJSONTag(pass, field, v[0])
				case "xml":
					if _, err := fakexml.StructFieldInfo(rt.Field(i)); err != nil {
						report.Report(pass, field.Tag, fmt.Sprintf("invalid XML tag: %s", err))
					}
					checkXMLTag(pass, field, v[0])
				}
			}
		}
	}
	code.Preorder(pass, fn, (*ast.StructType)(nil))
	return nil, nil
}

func checkJSONTag(pass *analysis.Pass, field *ast.Field, tag string) {
	if pass.Pkg.Path() == "encoding/json" || pass.Pkg.Path() == "encoding/json_test" {
		// don't flag malformed JSON tags in the encoding/json
		// package; it knows what it is doing, and it is testing
		// itself.
		return
	}
	//lint:ignore SA9003 TODO(dh): should we flag empty tags?
	if len(tag) == 0 {
	}
	fields := strings.Split(tag, ",")
	for _, r := range fields[0] {
		if !unicode.IsLetter(r) && !unicode.IsDigit(r) && !strings.ContainsRune("!#$%&()*+-./:<=>?@[]^_{|}~ ", r) {
			report.Report(pass, field.Tag, fmt.Sprintf("invalid JSON field name %q", fields[0]))
		}
	}
	var co, cs, ci int
	for _, s := range fields[1:] {
		switch s {
		case "omitempty":
			co++
		case "":
			// allow stuff like "-,"
		case "string":
			cs++
			// only for string, floating point, integer and bool
			tset := typeutil.NewTypeSet(pass.TypesInfo.TypeOf(field.Type))
			if len(tset.Terms) == 0 {
				// TODO(dh): improve message, call out the use of type parameters
				report.Report(pass, field.Tag, "the JSON string option only applies to fields of type string, floating point, integer or bool, or pointers to those")
				continue
			}
			for _, term := range tset.Terms {
				T := typeutil.Dereference(term.Type().Underlying())
				for _, term2 := range typeutil.NewTypeSet(T).Terms {
					basic, ok := term2.Type().Underlying().(*types.Basic)
					if !ok || (basic.Info()&(types.IsBoolean|types.IsInteger|types.IsFloat|types.IsString)) == 0 {
						// TODO(dh): improve message, show how we arrived at the type
						report.Report(pass, field.Tag, "the JSON string option only applies to fields of type string, floating point, integer or bool, or pointers to those")
					}
				}
			}
		case "inline":
			ci++
		default:
			report.Report(pass, field.Tag, fmt.Sprintf("unknown JSON option %q", s))
		}
	}
	if co > 1 {
		report.Report(pass, field.Tag, `duplicate JSON option "omitempty"`)
	}
	if cs > 1 {
		report.Report(pass, field.Tag, `duplicate JSON option "string"`)
	}
	if ci > 1 {
		report.Report(pass, field.Tag, `duplicate JSON option "inline"`)
	}
}

func checkXMLTag(pass *analysis.Pass, field *ast.Field, tag string) {
	//lint:ignore SA9003 TODO(dh): should we flag empty tags?
	if len(tag) == 0 {
	}
	fields := strings.Split(tag, ",")
	counts := map[string]int{}
	for _, s := range fields[1:] {
		switch s {
		case "attr", "chardata", "cdata", "innerxml", "comment":
			counts[s]++
		case "omitempty", "any":
			counts[s]++
		case "":
		default:
			report.Report(pass, field.Tag, fmt.Sprintf("invalid XML tag: unknown option %q", s))
		}
	}
	for k, v := range counts {
		if v > 1 {
			report.Report(pass, field.Tag, fmt.Sprintf("invalid XML tag: duplicate option %q", k))
		}
	}
}

func CheckImpossibleTypeAssertion(pass *analysis.Pass) (interface{}, error) {
	type entry struct {
		l, r *types.Func
	}

	msc := &pass.ResultOf[buildir.Analyzer].(*buildir.IR).Pkg.Prog.MethodSets
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, b := range fn.Blocks {
			for _, instr := range b.Instrs {
				assert, ok := instr.(*ir.TypeAssert)
				if !ok {
					continue
				}
				var wrong []entry
				left := assert.X.Type()
				right := assert.AssertedType
				righti, ok := right.Underlying().(*types.Interface)

				if !ok {
					// We only care about interface->interface
					// assertions. The Go compiler already catches
					// impossible interface->concrete assertions.
					continue
				}

				ms := msc.MethodSet(left)
				for i := 0; i < righti.NumMethods(); i++ {
					mr := righti.Method(i)
					sel := ms.Lookup(mr.Pkg(), mr.Name())
					if sel == nil {
						continue
					}
					ml := sel.Obj().(*types.Func)
					if types.AssignableTo(ml.Type(), mr.Type()) {
						continue
					}

					wrong = append(wrong, entry{ml, mr})
				}

				if len(wrong) != 0 {
					s := fmt.Sprintf("impossible type assertion; %s and %s contradict each other:",
						types.TypeString(left, types.RelativeTo(pass.Pkg)),
						types.TypeString(right, types.RelativeTo(pass.Pkg)))
					for _, e := range wrong {
						s += fmt.Sprintf("\n\twrong type for %s method", e.l.Name())
						s += fmt.Sprintf("\n\t\thave %s", e.l.Type())
						s += fmt.Sprintf("\n\t\twant %s", e.r.Type())
					}
					report.Report(pass, assert, s)
				}
			}
		}
	}
	return nil, nil
}

func checkWithValueKey(call *Call) {
	arg := call.Args[1]
	T := arg.Value.Value.Type()
	if T, ok := T.(*types.Basic); ok {
		arg.Invalid(
			fmt.Sprintf("should not use built-in type %s as key for value; define your own type to avoid collisions", T))
	}
	if !types.Comparable(T) {
		arg.Invalid(fmt.Sprintf("keys used with context.WithValue must be comparable, but type %s is not comparable", T))
	}
}

func CheckMaybeNil(pass *analysis.Pass) (interface{}, error) {
	// This is an extremely trivial check that doesn't try to reason
	// about control flow. That is, phis and sigmas do not propagate
	// any information. As such, we can flag this:
	//
	// 	_ = *x
	// 	if x == nil { return }
	//
	// but we cannot flag this:
	//
	// 	if x == nil { println(x) }
	// 	_ = *x
	//
	// but we can flag this, because the if's body doesn't use x:
	//
	// 	if x == nil { println("this is bad") }
	//  _ = *x
	//
	// nor many other variations of conditional uses of or assignments to x.
	//
	// However, even this trivial implementation finds plenty of
	// real-world bugs, such as dereference before nil pointer check,
	// or using t.Error instead of t.Fatal when encountering nil
	// pointers.
	//
	// On the flip side, our naive implementation avoids false positives in branches, such as
	//
	// 	if x != nil { _ = *x }
	//
	// due to the same lack of propagating information through sigma
	// nodes. x inside the branch will be independent of the x in the
	// nil pointer check.
	//
	//
	// We could implement a more powerful check, but then we'd be
	// getting false positives instead of false negatives because
	// we're incapable of deducing relationships between variables.
	// For example, a function might return a pointer and an error,
	// and the error being nil guarantees that the pointer is not nil.
	// Depending on the surrounding code, the pointer may still end up
	// being checked against nil in one place, and guarded by a check
	// on the error in another, which would lead to us marking some
	// loads as unsafe.
	//
	// Unfortunately, simply hard-coding the relationship between
	// return values wouldn't eliminate all false positives, either.
	// Many other more subtle relationships exist. An abridged example
	// from real code:
	//
	// if a == nil && b == nil { return }
	// c := fn(a)
	// if c != "" { _ = *a }
	//
	// where `fn` is guaranteed to return a non-empty string if a
	// isn't nil.
	//
	// We choose to err on the side of false negatives.

	isNilConst := func(v ir.Value) bool {
		if typeutil.IsPointerLike(v.Type()) {
			if k, ok := v.(*ir.Const); ok {
				return k.IsNil()
			}
		}
		return false
	}

	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		maybeNil := map[ir.Value]ir.Instruction{}
		for _, b := range fn.Blocks {
			for _, instr := range b.Instrs {
				// Originally we looked at all ir.BinOp, but that would lead to calls like 'assert(x != nil)' causing false positives.
				// Restrict ourselves to actual if statements, as these are more likely to affect control flow in a way we can observe.
				if instr, ok := instr.(*ir.If); ok {
					if cond, ok := instr.Cond.(*ir.BinOp); ok {
						var ptr ir.Value
						if isNilConst(cond.X) {
							ptr = cond.Y
						} else if isNilConst(cond.Y) {
							ptr = cond.X
						}
						maybeNil[ptr] = cond
					}
				}
			}
		}

		for _, b := range fn.Blocks {
			for _, instr := range b.Instrs {
				var ptr ir.Value
				switch instr := instr.(type) {
				case *ir.Load:
					ptr = instr.X
				case *ir.Store:
					ptr = instr.Addr
				case *ir.IndexAddr:
					ptr = instr.X
					if typeutil.All(ptr.Type(), func(term *typeparams.Term) bool {
						if _, ok := term.Type().Underlying().(*types.Slice); ok {
							return true
						}
						return false
					}) {
						// indexing a nil slice does not cause a nil pointer panic
						//
						// Note: This also works around the bad lowering of range loops over slices
						// (https://github.com/dominikh/go-tools/issues/1053)
						continue
					}
				case *ir.FieldAddr:
					ptr = instr.X
				}
				if ptr != nil {
					switch ptr.(type) {
					case *ir.Alloc, *ir.FieldAddr, *ir.IndexAddr:
						// these cannot be nil
						continue
					}
					if r, ok := maybeNil[ptr]; ok {
						report.Report(pass, instr, "possible nil pointer dereference",
							report.Related(r, "this check suggests that the pointer can be nil"))
					}
				}
			}
		}
	}

	return nil, nil
}

var checkAddressIsNilQ = pattern.MustParse(
	`(BinaryExpr
		(UnaryExpr "&" _)
		(Or "==" "!=")
		(Builtin "nil"))`)

func CheckAddressIsNil(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		_, ok := code.Match(pass, checkAddressIsNilQ, node)
		if !ok {
			return
		}
		report.Report(pass, node, "the address of a variable cannot be nil")
	}
	code.Preorder(pass, fn, (*ast.BinaryExpr)(nil))
	return nil, nil
}

var (
	checkFixedLengthTypeShiftQ = pattern.MustParse(`
		(Or
			(AssignStmt _ (Or ">>=" "<<=") _)
			(BinaryExpr _ (Or ">>" "<<") _))
	`)
)

func CheckStaticBitShift(pass *analysis.Pass) (interface{}, error) {
	isDubiousShift := func(x, y ast.Expr) (int64, int64, bool) {
		typ, ok := pass.TypesInfo.TypeOf(x).Underlying().(*types.Basic)
		if !ok {
			return 0, 0, false
		}
		switch typ.Kind() {
		case types.Int8, types.Int16, types.Int32, types.Int64,
			types.Uint8, types.Uint16, types.Uint32, types.Uint64:
			// We're only interested in fixed–size types.
		default:
			return 0, 0, false
		}

		const bitsInByte = 8
		typeBits := pass.TypesSizes.Sizeof(typ) * bitsInByte

		shiftLength, ok := code.ExprToInt(pass, y)
		if !ok {
			return 0, 0, false
		}

		return typeBits, shiftLength, shiftLength >= typeBits
	}

	fn := func(node ast.Node) {
		if _, ok := code.Match(pass, checkFixedLengthTypeShiftQ, node); !ok {
			return
		}

		switch e := node.(type) {
		case *ast.AssignStmt:
			if size, shift, yes := isDubiousShift(e.Lhs[0], e.Rhs[0]); yes {
				report.Report(pass, e, fmt.Sprintf("shifting %d-bit value by %d bits will always clear it", size, shift))
			}
		case *ast.BinaryExpr:
			if size, shift, yes := isDubiousShift(e.X, e.Y); yes {
				report.Report(pass, e, fmt.Sprintf("shifting %d-bit value by %d bits will always clear it", size, shift))
			}
		}
	}
	code.Preorder(pass, fn, (*ast.AssignStmt)(nil), (*ast.BinaryExpr)(nil))

	return nil, nil
}

func findSliceLenChecks(pass *analysis.Pass) {
	// mark all function parameters that have to be of even length
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, b := range fn.Blocks {
			// all paths go through this block
			if !b.Dominates(fn.Exit) {
				continue
			}

			// if foo % 2 != 0
			ifi, ok := b.Control().(*ir.If)
			if !ok {
				continue
			}
			cmp, ok := ifi.Cond.(*ir.BinOp)
			if !ok {
				continue
			}
			var needle uint64
			switch cmp.Op {
			case token.NEQ:
				// look for != 0
				needle = 0
			case token.EQL:
				// look for == 1
				needle = 1
			default:
				continue
			}

			rem, ok1 := cmp.X.(*ir.BinOp)
			k, ok2 := cmp.Y.(*ir.Const)
			if ok1 != ok2 {
				continue
			}
			if !ok1 {
				rem, ok1 = cmp.Y.(*ir.BinOp)
				k, ok2 = cmp.X.(*ir.Const)
			}
			if !ok1 || !ok2 || rem.Op != token.REM || k.Value.Kind() != constant.Int || k.Uint64() != needle {
				continue
			}
			k, ok = rem.Y.(*ir.Const)
			if !ok || k.Value.Kind() != constant.Int || k.Uint64() != 2 {
				continue
			}

			// if len(foo) % 2 != 0
			call, ok := rem.X.(*ir.Call)
			if !ok || !irutil.IsCallTo(call.Common(), "len") {
				continue
			}

			// we're checking the length of a parameter that is a slice
			// TODO(dh): support parameters that have flown through sigmas and phis
			param, ok := call.Call.Args[0].(*ir.Parameter)
			if !ok {
				continue
			}
			if !typeutil.All(param.Type(), typeutil.IsSlice) {
				continue
			}

			// if len(foo) % 2 != 0 then panic
			if _, ok := b.Succs[0].Control().(*ir.Panic); !ok {
				continue
			}

			pass.ExportObjectFact(param.Object(), new(evenElements))
		}
	}
}

func findIndirectSliceLenChecks(pass *analysis.Pass) {
	seen := map[*ir.Function]struct{}{}

	var doFunction func(fn *ir.Function)
	doFunction = func(fn *ir.Function) {
		if _, ok := seen[fn]; ok {
			return
		}
		seen[fn] = struct{}{}

		for _, b := range fn.Blocks {
			// all paths go through this block
			if !b.Dominates(fn.Exit) {
				continue
			}

			for _, instr := range b.Instrs {
				call, ok := instr.(*ir.Call)
				if !ok {
					continue
				}
				callee := call.Call.StaticCallee()
				if callee == nil {
					continue
				}

				if callee.Pkg == fn.Pkg || callee.Pkg == nil {
					doFunction(callee)
				}

				for argi, arg := range call.Call.Args {
					if callee.Signature.Recv() != nil {
						if argi == 0 {
							continue
						}
						argi--
					}

					// TODO(dh): support parameters that have flown through length-preserving instructions
					param, ok := arg.(*ir.Parameter)
					if !ok {
						continue
					}
					if !typeutil.All(param.Type(), typeutil.IsSlice) {
						continue
					}

					// We can't use callee.Params to look up the
					// parameter, because Params is not populated for
					// external functions. In our modular analysis.
					// any function in any package that isn't the
					// current package is considered "external", as it
					// has been loaded from export data only.
					sigParams := callee.Signature.Params()

					if !pass.ImportObjectFact(sigParams.At(argi), new(evenElements)) {
						continue
					}
					pass.ExportObjectFact(param.Object(), new(evenElements))
				}
			}
		}
	}

	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		doFunction(fn)
	}
}

func findSliceLength(v ir.Value) int {
	// TODO(dh): VRP would help here

	v = irutil.Flatten(v)
	val := func(v ir.Value) int {
		if v, ok := v.(*ir.Const); ok {
			return int(v.Int64())
		}
		return -1
	}
	switch v := v.(type) {
	case *ir.Slice:
		low := 0
		high := -1
		if v.Low != nil {
			low = val(v.Low)
		}
		if v.High != nil {
			high = val(v.High)
		} else {
			switch vv := v.X.(type) {
			case *ir.Alloc:
				high = int(typeutil.Dereference(vv.Type()).Underlying().(*types.Array).Len())
			case *ir.Slice:
				high = findSliceLength(vv)
			}
		}
		if low == -1 || high == -1 {
			return -1
		}
		return high - low
	default:
		return -1
	}
}

type evenElements struct{}

func (evenElements) AFact() {}

func (evenElements) String() string { return "needs even elements" }

func flagSliceLens(pass *analysis.Pass) {
	var tag evenElements

	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, b := range fn.Blocks {
			for _, instr := range b.Instrs {
				call, ok := instr.(ir.CallInstruction)
				if !ok {
					continue
				}
				callee := call.Common().StaticCallee()
				if callee == nil {
					continue
				}
				for argi, arg := range call.Common().Args {
					if callee.Signature.Recv() != nil {
						if argi == 0 {
							continue
						}
						argi--
					}

					_, ok := arg.Type().Underlying().(*types.Slice)
					if !ok {
						continue
					}
					param := callee.Signature.Params().At(argi)
					if !pass.ImportObjectFact(param, &tag) {
						continue
					}

					// TODO handle stubs

					// we know the argument has to have even length.
					// now let's try to find its length
					if n := findSliceLength(arg); n > -1 && n%2 != 0 {
						src := call.Source().(*ast.CallExpr).Args[argi]
						sig := call.Common().Signature()
						var label string
						if argi == sig.Params().Len()-1 && sig.Variadic() {
							label = "variadic argument"
						} else {
							label = "argument"
						}
						// Note that param.Name() is guaranteed to not
						// be empty, otherwise the function couldn't
						// have enforced its length.
						report.Report(pass, src, fmt.Sprintf("%s %q is expected to have even number of elements, but has %d elements", label, param.Name(), n))
					}
				}
			}
		}
	}
}

func CheckEvenSliceLength(pass *analysis.Pass) (interface{}, error) {
	findSliceLenChecks(pass)
	findIndirectSliceLenChecks(pass)
	flagSliceLens(pass)

	return nil, nil
}

func CheckTypedNilInterface(pass *analysis.Pass) (interface{}, error) {
	// The comparison 'fn() == nil' can never be true if fn() returns
	// an interface value and only returns typed nils. This is usually
	// a mistake in the function itself, but all we can say for
	// certain is that the comparison is pointless.
	//
	// Flag results if no untyped nils are being returned, but either
	// known typed nils, or typed unknown nilness are being returned.

	irpkg := pass.ResultOf[buildir.Analyzer].(*buildir.IR)
	typedness := pass.ResultOf[typedness.Analysis].(*typedness.Result)
	nilness := pass.ResultOf[nilness.Analysis].(*nilness.Result)
	for _, fn := range irpkg.SrcFuncs {
		for _, b := range fn.Blocks {
			for _, instr := range b.Instrs {
				binop, ok := instr.(*ir.BinOp)
				if !ok || !(binop.Op == token.EQL || binop.Op == token.NEQ) {
					continue
				}
				if _, ok := binop.X.Type().Underlying().(*types.Interface); !ok || typeparams.IsTypeParam(binop.X.Type()) {
					// TODO support swapped X and Y
					continue
				}

				k, ok := binop.Y.(*ir.Const)
				if !ok || !k.IsNil() {
					// if binop.X is an interface, then binop.Y can
					// only be a Const if its untyped. A typed nil
					// constant would first be passed to
					// MakeInterface.
					continue
				}

				var idx int
				var obj *types.Func
				switch x := irutil.Flatten(binop.X).(type) {
				case *ir.Call:
					callee := x.Call.StaticCallee()
					if callee == nil {
						continue
					}
					obj, _ = callee.Object().(*types.Func)
					idx = 0
				case *ir.Extract:
					call, ok := irutil.Flatten(x.Tuple).(*ir.Call)
					if !ok {
						continue
					}
					callee := call.Call.StaticCallee()
					if callee == nil {
						continue
					}
					obj, _ = callee.Object().(*types.Func)
					idx = x.Index
				case *ir.MakeInterface:
					var qualifier string
					switch binop.Op {
					case token.EQL:
						qualifier = "never"
					case token.NEQ:
						qualifier = "always"
					default:
						panic("unreachable")
					}

					terms, err := typeparams.NormalTerms(x.X.Type())
					if len(terms) == 0 || err != nil {
						// Type is a type parameter with no type terms (or we couldn't determine the terms). Such a type
						// _can_ be nil when put in an interface value.
						continue
					}

					if report.HasRange(x.X) {
						report.Report(pass, binop, fmt.Sprintf("this comparison is %s true", qualifier),
							report.Related(x.X, "the lhs of the comparison gets its value from here and has a concrete type"))
					} else {
						// we can't generate related information for this, so make the diagnostic itself slightly more useful
						report.Report(pass, binop, fmt.Sprintf("this comparison is %s true; the lhs of the comparison has been assigned a concretely typed value", qualifier))
					}
					continue
				}
				if obj == nil {
					continue
				}

				isNil, onlyGlobal := nilness.MayReturnNil(obj, idx)
				if typedness.MustReturnTyped(obj, idx) && isNil && !onlyGlobal && !code.IsInTest(pass, binop) {
					// Don't flag these comparisons in tests. Tests
					// may be explicitly enforcing the invariant that
					// a value isn't nil.

					var qualifier string
					switch binop.Op {
					case token.EQL:
						qualifier = "never"
					case token.NEQ:
						qualifier = "always"
					default:
						panic("unreachable")
					}
					report.Report(pass, binop, fmt.Sprintf("this comparison is %s true", qualifier),
						// TODO support swapped X and Y
						report.Related(binop.X, fmt.Sprintf("the lhs of the comparison is the %s return value of this function call", report.Ordinal(idx+1))),
						report.Related(obj, fmt.Sprintf("%s never returns a nil interface value", typeutil.FuncName(obj))))
				}
			}
		}
	}

	return nil, nil
}

var builtinLessThanZeroQ = pattern.MustParse(`
	(Or
		(BinaryExpr
			(IntegerLiteral "0")
			">"
			(CallExpr builtin@(Builtin (Or "len" "cap")) _))
		(BinaryExpr
			(CallExpr builtin@(Builtin (Or "len" "cap")) _)
			"<"
			(IntegerLiteral "0")))
`)

func CheckBuiltinZeroComparison(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		matcher, ok := code.Match(pass, builtinLessThanZeroQ, node)
		if !ok {
			return
		}

		builtin := matcher.State["builtin"].(*ast.Ident)
		report.Report(pass, node, fmt.Sprintf("builtin function %s does not return negative values", builtin.Name))
	}
	code.Preorder(pass, fn, (*ast.BinaryExpr)(nil))

	return nil, nil
}

var integerDivisionQ = pattern.MustParse(`(BinaryExpr (IntegerLiteral _) "/" (IntegerLiteral _))`)

func CheckIntegerDivisionEqualsZero(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		_, ok := code.Match(pass, integerDivisionQ, node)
		if !ok {
			return
		}

		val := constant.ToInt(pass.TypesInfo.Types[node.(ast.Expr)].Value)
		if v, ok := constant.Uint64Val(val); ok && v == 0 {
			report.Report(pass, node, fmt.Sprintf("the integer division '%s' results in zero", report.Render(pass, node)))
		}

		// TODO: we could offer a suggested fix here, but I am not
		// sure what it should be. There are many options to choose
		// from.

		// Note: we experimented with flagging divisions that truncate
		// (e.g. 4 / 3), but it ran into false positives in Go's
		// 'time' package, which does this, deliberately:
		//
		//   unixToInternal int64 = (1969*365 + 1969/4 - 1969/100 + 1969/400) * secondsPerDay
		//
		// The check also found a real bug in other code, but I don't
		// think we can outright ban this kind of division.
	}
	code.Preorder(pass, fn, (*ast.BinaryExpr)(nil))

	return nil, nil
}

func CheckIneffectiveFieldAssignments(pass *analysis.Pass) (interface{}, error) {
	// The analysis only considers the receiver and its first level
	// fields. It doesn't look at other parameters, nor at nested
	// fields.
	//
	// The analysis does not detect all kinds of dead stores, only
	// those of fields that are never read after the write. That is,
	// we do not flag 'a.x = 1; a.x = 2; _ = a.x'. We might explore
	// this again if we add support for SROA to go/ir and implement
	// https://github.com/dominikh/go-tools/issues/191.

	irpkg := pass.ResultOf[buildir.Analyzer].(*buildir.IR)
fnLoop:
	for _, fn := range irpkg.SrcFuncs {
		if recv := fn.Signature.Recv(); recv == nil {
			continue
		} else if _, ok := recv.Type().Underlying().(*types.Struct); !ok {
			continue
		}

		recv := fn.Params[0]
		refs := irutil.FilterDebug(*recv.Referrers())
		if len(refs) != 1 {
			continue
		}
		store, ok := refs[0].(*ir.Store)
		if !ok {
			continue
		}
		alloc, ok := store.Addr.(*ir.Alloc)
		if !ok || alloc.Heap {
			continue
		}

		reads := map[int][]ir.Instruction{}
		writes := map[int][]ir.Instruction{}
		for _, ref := range *alloc.Referrers() {
			switch ref := ref.(type) {
			case *ir.FieldAddr:
				for _, refref := range *ref.Referrers() {
					switch refref.(type) {
					case *ir.Store:
						writes[ref.Field] = append(writes[ref.Field], refref)
					case *ir.Load:
						reads[ref.Field] = append(reads[ref.Field], refref)
					case *ir.DebugRef:
						continue
					default:
						// this should be safe… if the field address
						// escapes, then alloc.Heap will be true.
						// there should be no instructions left that,
						// given this FieldAddr, without escaping, can
						// effect a load or store.
						continue
					}
				}
			case *ir.Store:
				// we could treat this as a store to every field, but
				// we don't want to decide the semantics of partial
				// struct initializers. should `v = t{x: 1}` also mark
				// v.y as being written to?
				if ref != store {
					continue fnLoop
				}
			case *ir.Load:
				// a load of the entire struct loads every field
				for i := 0; i < recv.Type().Underlying().(*types.Struct).NumFields(); i++ {
					reads[i] = append(reads[i], ref)
				}
			case *ir.DebugRef:
				continue
			default:
				continue fnLoop
			}
		}

		offset := func(instr ir.Instruction) int {
			for i, other := range instr.Block().Instrs {
				if instr == other {
					return i
				}
			}
			panic("couldn't find instruction in its block")
		}

		for field, ws := range writes {
			rs := reads[field]
		wLoop:
			for _, w := range ws {
				for _, r := range rs {
					if w.Block() == r.Block() {
						if offset(r) > offset(w) {
							// found a reachable read of our write
							continue wLoop
						}
					} else if irutil.Reachable(w.Block(), r.Block()) {
						// found a reachable read of our write
						continue wLoop
					}
				}
				fieldName := recv.Type().Underlying().(*types.Struct).Field(field).Name()
				report.Report(pass, w, fmt.Sprintf("ineffective assignment to field %s.%s", recv.Type().(*types.Named).Obj().Name(), fieldName))
			}
		}
	}
	return nil, nil
}

var negativeZeroFloatQ = pattern.MustParse(`
	(Or
		(UnaryExpr
			"-"
			(BasicLit "FLOAT" "0.0"))

		(UnaryExpr
			"-"
			(CallExpr conv@(Object (Or "float32" "float64")) lit@(Or (BasicLit "INT" "0") (BasicLit "FLOAT" "0.0"))))

		(CallExpr
			conv@(Object (Or "float32" "float64"))
			(UnaryExpr "-" lit@(BasicLit "INT" "0"))))`)

func CheckNegativeZeroFloat(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		m, ok := code.Match(pass, negativeZeroFloatQ, node)
		if !ok {
			return
		}

		if conv, ok := m.State["conv"].(*types.TypeName); ok {
			var replacement string
			// TODO(dh): how does this handle type aliases?
			if conv.Name() == "float32" {
				replacement = `float32(math.Copysign(0, -1))`
			} else {
				replacement = `math.Copysign(0, -1)`
			}
			report.Report(pass, node,
				fmt.Sprintf("in Go, the floating-point expression '%s' is the same as '%s(%s)', it does not produce a negative zero",
					report.Render(pass, node),
					conv.Name(),
					report.Render(pass, m.State["lit"])),
				report.Fixes(edit.Fix("use math.Copysign to create negative zero", edit.ReplaceWithString(node, replacement))))
		} else {
			const replacement = `math.Copysign(0, -1)`
			report.Report(pass, node,
				"in Go, the floating-point literal '-0.0' is the same as '0.0', it does not produce a negative zero",
				report.Fixes(edit.Fix("use math.Copysign to create negative zero", edit.ReplaceWithString(node, replacement))))
		}
	}
	code.Preorder(pass, fn, (*ast.UnaryExpr)(nil), (*ast.CallExpr)(nil))
	return nil, nil
}

var ineffectiveURLQueryAddQ = pattern.MustParse(`(CallExpr (SelectorExpr (CallExpr (SelectorExpr recv (Ident "Query")) []) (Ident meth)) _)`)

func CheckIneffectiveURLQueryModification(pass *analysis.Pass) (interface{}, error) {
	// TODO(dh): We could make this check more complex and detect
	// pointless modifications of net/url.Values in general, but that
	// requires us to get the state machine correct, else we'll cause
	// false positives.

	fn := func(node ast.Node) {
		m, ok := code.Match(pass, ineffectiveURLQueryAddQ, node)
		if !ok {
			return
		}
		if !code.IsOfType(pass, m.State["recv"].(ast.Expr), "*net/url.URL") {
			return
		}
		switch m.State["meth"].(string) {
		case "Add", "Del", "Set":
		default:
			return
		}
		report.Report(pass, node, "(*net/url.URL).Query returns a copy, modifying it doesn't change the URL")
	}
	code.Preorder(pass, fn, (*ast.CallExpr)(nil))
	return nil, nil
}

func CheckBadRemoveAll(pass *analysis.Pass) (interface{}, error) {
	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
		for _, b := range fn.Blocks {
			for _, instr := range b.Instrs {
				call, ok := instr.(ir.CallInstruction)
				if !ok {
					continue
				}
				if !irutil.IsCallTo(call.Common(), "os.RemoveAll") {
					continue
				}

				kind := ""
				ex := ""
				callName := ""
				arg := irutil.Flatten(call.Common().Args[0])
				switch arg := arg.(type) {
				case *ir.Call:
					callName = irutil.CallName(&arg.Call)
					if callName != "os.TempDir" {
						continue
					}
					kind = "temporary"
					ex = os.TempDir()
				case *ir.Extract:
					if arg.Index != 0 {
						continue
					}
					first, ok := arg.Tuple.(*ir.Call)
					if !ok {
						continue
					}
					callName = irutil.CallName(&first.Call)
					switch callName {
					case "os.UserCacheDir":
						kind = "cache"
						ex, _ = os.UserCacheDir()
					case "os.UserConfigDir":
						kind = "config"
						ex, _ = os.UserConfigDir()
					case "os.UserHomeDir":
						kind = "home"
						ex, _ = os.UserHomeDir()
					default:
						continue
					}
				default:
					continue
				}

				if ex == "" {
					report.Report(pass, call, fmt.Sprintf("this call to os.RemoveAll deletes the user's entire %s directory, not a subdirectory therein", kind),
						report.Related(arg, fmt.Sprintf("this call to %s returns the user's %s directory", callName, kind)))
				} else {
					report.Report(pass, call, fmt.Sprintf("this call to os.RemoveAll deletes the user's entire %s directory, not a subdirectory therein", kind),
						report.Related(arg, fmt.Sprintf("this call to %s returns the user's %s directory, for example %s", callName, kind, ex)))
				}
			}
		}
	}
	return nil, nil
}

var moduloOneQ = pattern.MustParse(`(BinaryExpr _ "%" (IntegerLiteral "1"))`)

func CheckModuloOne(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		_, ok := code.Match(pass, moduloOneQ, node)
		if !ok {
			return
		}
		report.Report(pass, node, "x % 1 is always zero")
	}
	code.Preorder(pass, fn, (*ast.BinaryExpr)(nil))
	return nil, nil
}

var typeAssertionShadowingElseQ = pattern.MustParse(`(IfStmt (AssignStmt [obj@(Ident _) ok@(Ident _)] ":=" assert@(TypeAssertExpr obj _)) ok _ elseBranch)`)

func CheckTypeAssertionShadowingElse(pass *analysis.Pass) (interface{}, error) {
	// TODO(dh): without the IR-based verification, this check is able
	// to find more bugs, but also more prone to false positives. It
	// would be a good candidate for the 'codereview' category of
	// checks.

	irpkg := pass.ResultOf[buildir.Analyzer].(*buildir.IR).Pkg
	fn := func(node ast.Node) {
		m, ok := code.Match(pass, typeAssertionShadowingElseQ, node)
		if !ok {
			return
		}
		shadow := pass.TypesInfo.ObjectOf(m.State["obj"].(*ast.Ident))
		shadowed := m.State["assert"].(*ast.TypeAssertExpr).X

		path, exact := astutil.PathEnclosingInterval(code.File(pass, shadow), shadow.Pos(), shadow.Pos())
		if !exact {
			// TODO(dh): when can this happen?
			return
		}
		irfn := ir.EnclosingFunction(irpkg, path)

		shadoweeIR, isAddr := irfn.ValueForExpr(m.State["obj"].(*ast.Ident))
		if shadoweeIR == nil || isAddr {
			// TODO(dh): is this possible?
			return
		}

		var branch ast.Node
		switch br := m.State["elseBranch"].(type) {
		case ast.Node:
			branch = br
		case []ast.Stmt:
			branch = &ast.BlockStmt{List: br}
		case nil:
			return
		default:
			panic(fmt.Sprintf("unexpected type %T", br))
		}

		ast.Inspect(branch, func(node ast.Node) bool {
			ident, ok := node.(*ast.Ident)
			if !ok {
				return true
			}
			if pass.TypesInfo.ObjectOf(ident) != shadow {
				return true
			}

			v, isAddr := irfn.ValueForExpr(ident)
			if v == nil || isAddr {
				return true
			}
			if irutil.Flatten(v) != shadoweeIR {
				// Same types.Object, but different IR value. This
				// either means that the variable has been
				// assigned to since the type assertion, or that
				// the variable has escaped to the heap. Either
				// way, we shouldn't flag reads of it.
				return true
			}

			report.Report(pass, ident,
				fmt.Sprintf("%s refers to the result of a failed type assertion and is a zero value, not the value that was being type-asserted", report.Render(pass, ident)),
				report.Related(shadow, "this is the variable being read"),
				report.Related(shadowed, "this is the variable being shadowed"))
			return true
		})
	}
	code.Preorder(pass, fn, (*ast.IfStmt)(nil))
	return nil, nil
}

var ineffectiveSortQ = pattern.MustParse(`(AssignStmt target@(Ident _) "=" (CallExpr typ@(Function (Or "sort.Float64Slice" "sort.IntSlice" "sort.StringSlice")) [target]))`)

func CheckIneffectiveSort(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		m, ok := code.Match(pass, ineffectiveSortQ, node)
		if !ok {
			return
		}

		_, ok = pass.TypesInfo.TypeOf(m.State["target"].(ast.Expr)).(*types.Slice)
		if !ok {
			// Avoid flagging 'x = sort.StringSlice(x)' where TypeOf(x) == sort.StringSlice
			return
		}

		var alternative string
		typeName := types.TypeString(m.State["typ"].(*types.TypeName).Type(), nil)
		switch typeName {
		case "sort.Float64Slice":
			alternative = "Float64s"
		case "sort.IntSlice":
			alternative = "Ints"
		case "sort.StringSlice":
			alternative = "Strings"
		default:
			panic(fmt.Sprintf("unreachable: %q", typeName))
		}

		r := &ast.CallExpr{
			Fun: &ast.SelectorExpr{
				X:   &ast.Ident{Name: "sort"},
				Sel: &ast.Ident{Name: alternative},
			},
			Args: []ast.Expr{m.State["target"].(ast.Expr)},
		}

		report.Report(pass, node,
			fmt.Sprintf("%s is a type, not a function, and %s doesn't sort your values; consider using sort.%s instead",
				typeName,
				report.Render(pass, node.(*ast.AssignStmt).Rhs[0]),
				alternative),
			report.Fixes(edit.Fix(fmt.Sprintf("replace with call to sort.%s", alternative), edit.ReplaceWithNode(pass.Fset, node, r))))
	}
	code.Preorder(pass, fn, (*ast.AssignStmt)(nil))
	return nil, nil
}

var ineffectiveRandIntQ = pattern.MustParse(`
	(CallExpr
		(Function
			name@(Or
				"math/rand.Int31n"
				"math/rand.Int63n"
				"math/rand.Intn"
				"(*math/rand.Rand).Int31n"
				"(*math/rand.Rand).Int63n"
				"(*math/rand.Rand).Intn"))
		[(IntegerLiteral "1")])`)

func CheckIneffectiveRandInt(pass *analysis.Pass) (interface{}, error) {
	fn := func(node ast.Node) {
		m, ok := code.Match(pass, ineffectiveRandIntQ, node)
		if !ok {
			return
		}

		report.Report(pass, node,
			fmt.Sprintf("%s(n) generates a random value 0 <= x < n; that is, the generated values don't include n; %s therefore always returns 0",
				m.State["name"], report.Render(pass, node)))
	}

	code.Preorder(pass, fn, (*ast.CallExpr)(nil))
	return nil, nil
}

var allocationNilCheckQ = pattern.MustParse(`(IfStmt _ cond@(BinaryExpr lhs op@(Or "==" "!=") (Builtin "nil")) _ _)`)

func CheckAllocationNilCheck(pass *analysis.Pass) (interface{}, error) {
	irpkg := pass.ResultOf[buildir.Analyzer].(*buildir.IR).Pkg
	fn := func(node ast.Node) {
		m, ok := code.Match(pass, allocationNilCheckQ, node)
		if !ok {
			return
		}
		cond := m.State["cond"].(ast.Node)
		if _, ok := code.Match(pass, checkAddressIsNilQ, cond); ok {
			// Don't duplicate diagnostics reported by SA4022
			return
		}
		lhs := m.State["lhs"].(ast.Expr)
		path, exact := astutil.PathEnclosingInterval(code.File(pass, lhs), lhs.Pos(), lhs.Pos())
		if !exact {
			// TODO(dh): when can this happen?
			return
		}
		irfn := ir.EnclosingFunction(irpkg, path)
		v, isAddr := irfn.ValueForExpr(lhs)
		if isAddr {
			return
		}

		seen := map[ir.Value]struct{}{}
		var values []ir.Value
		var neverNil func(v ir.Value, track bool) bool
		neverNil = func(v ir.Value, track bool) bool {
			if _, ok := seen[v]; ok {
				return true
			}
			seen[v] = struct{}{}
			switch v := v.(type) {
			case *ir.MakeClosure, *ir.Function:
				if track {
					values = append(values, v)
				}
				return true
			case *ir.MakeChan, *ir.MakeMap, *ir.MakeSlice, *ir.Alloc:
				if track {
					values = append(values, v)
				}
				return true
			case *ir.Slice:
				if track {
					values = append(values, v)
				}
				return neverNil(v.X, false)
			case *ir.FieldAddr:
				if track {
					values = append(values, v)
				}
				return neverNil(v.X, false)
			case *ir.Sigma:
				return neverNil(v.X, true)
			case *ir.Phi:
				for _, e := range v.Edges {
					if !neverNil(e, true) {
						return false
					}
				}
				return true
			default:
				return false
			}
		}

		if !neverNil(v, true) {
			return
		}

		var qualifier string
		if op := m.State["op"].(token.Token); op == token.EQL {
			qualifier = "never"
		} else {
			qualifier = "always"
		}
		fallback := fmt.Sprintf("this nil check is %s true", qualifier)

		sort.Slice(values, func(i, j int) bool { return values[i].Pos() < values[j].Pos() })

		if ident, ok := m.State["lhs"].(*ast.Ident); ok {
			if _, ok := pass.TypesInfo.ObjectOf(ident).(*types.Var); ok {
				var opts []report.Option
				if v.Parent() == irfn {
					if len(values) == 1 {
						opts = append(opts, report.Related(values[0], fmt.Sprintf("this is the value of %s", ident.Name)))
					} else {
						for _, vv := range values {
							opts = append(opts, report.Related(vv, fmt.Sprintf("this is one of the value of %s", ident.Name)))
						}
					}
				}

				switch v.(type) {
				case *ir.MakeClosure, *ir.Function:
					report.Report(pass, cond, "the checked variable contains a function and is never nil; did you mean to call it?", opts...)
				default:
					report.Report(pass, cond, fallback, opts...)
				}
			} else {
				if _, ok := v.(*ir.Function); ok {
					report.Report(pass, cond, "functions are never nil; did you mean to call it?")
				} else {
					report.Report(pass, cond, fallback)
				}
			}
		} else {
			if _, ok := v.(*ir.Function); ok {
				report.Report(pass, cond, "functions are never nil; did you mean to call it?")
			} else {
				report.Report(pass, cond, fallback)
			}
		}

	}
	code.Preorder(pass, fn, (*ast.IfStmt)(nil))
	return nil, nil
}