staticcheck (#313)

* CI: use staticcheck for linting

This commit switches the linter for Go code from golint to staticcheck.
Golint has been deprecated since last year and staticcheck is a
recommended replacement.

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

* revendor

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

* cmd,pkg: fix lint warnings

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

vendor/honnef.co/go/tools/analysis/facts/deprecated.go

@@ -0,0 +1,145 @@
+package facts
+
+import (
+	"go/ast"
+	"go/token"
+	"go/types"
+	"reflect"
+	"strings"
+
+	"golang.org/x/tools/go/analysis"
+)
+
+type IsDeprecated struct{ Msg string }
+
+func (*IsDeprecated) AFact()           {}
+func (d *IsDeprecated) String() string { return "Deprecated: " + d.Msg }
+
+type DeprecatedResult struct {
+	Objects  map[types.Object]*IsDeprecated
+	Packages map[*types.Package]*IsDeprecated
+}
+
+var Deprecated = &analysis.Analyzer{
+	Name:       "fact_deprecated",
+	Doc:        "Mark deprecated objects",
+	Run:        deprecated,
+	FactTypes:  []analysis.Fact{(*IsDeprecated)(nil)},
+	ResultType: reflect.TypeOf(DeprecatedResult{}),
+}
+
+func deprecated(pass *analysis.Pass) (interface{}, error) {
+	var names []*ast.Ident
+
+	extractDeprecatedMessage := func(docs []*ast.CommentGroup) string {
+		for _, doc := range docs {
+			if doc == nil {
+				continue
+			}
+			parts := strings.Split(doc.Text(), "\n\n")
+			for _, part := range parts {
+				if !strings.HasPrefix(part, "Deprecated: ") {
+					continue
+				}
+				alt := part[len("Deprecated: "):]
+				alt = strings.Replace(alt, "\n", " ", -1)
+				return alt
+			}
+		}
+		return ""
+	}
+	doDocs := func(names []*ast.Ident, docs []*ast.CommentGroup) {
+		alt := extractDeprecatedMessage(docs)
+		if alt == "" {
+			return
+		}
+
+		for _, name := range names {
+			obj := pass.TypesInfo.ObjectOf(name)
+			pass.ExportObjectFact(obj, &IsDeprecated{alt})
+		}
+	}
+
+	var docs []*ast.CommentGroup
+	for _, f := range pass.Files {
+		docs = append(docs, f.Doc)
+	}
+	if alt := extractDeprecatedMessage(docs); alt != "" {
+		// Don't mark package syscall as deprecated, even though
+		// it is. A lot of people still use it for simple
+		// constants like SIGKILL, and I am not comfortable
+		// telling them to use x/sys for that.
+		if pass.Pkg.Path() != "syscall" {
+			pass.ExportPackageFact(&IsDeprecated{alt})
+		}
+	}
+
+	docs = docs[:0]
+	for _, f := range pass.Files {
+		fn := func(node ast.Node) bool {
+			if node == nil {
+				return true
+			}
+			var ret bool
+			switch node := node.(type) {
+			case *ast.GenDecl:
+				switch node.Tok {
+				case token.TYPE, token.CONST, token.VAR:
+					docs = append(docs, node.Doc)
+					return true
+				default:
+					return false
+				}
+			case *ast.FuncDecl:
+				docs = append(docs, node.Doc)
+				names = []*ast.Ident{node.Name}
+				ret = false
+			case *ast.TypeSpec:
+				docs = append(docs, node.Doc)
+				names = []*ast.Ident{node.Name}
+				ret = true
+			case *ast.ValueSpec:
+				docs = append(docs, node.Doc)
+				names = node.Names
+				ret = false
+			case *ast.File:
+				return true
+			case *ast.StructType:
+				for _, field := range node.Fields.List {
+					doDocs(field.Names, []*ast.CommentGroup{field.Doc})
+				}
+				return false
+			case *ast.InterfaceType:
+				for _, field := range node.Methods.List {
+					doDocs(field.Names, []*ast.CommentGroup{field.Doc})
+				}
+				return false
+			default:
+				return false
+			}
+			if len(names) == 0 || len(docs) == 0 {
+				return ret
+			}
+			doDocs(names, docs)
+
+			docs = docs[:0]
+			names = nil
+			return ret
+		}
+		ast.Inspect(f, fn)
+	}
+
+	out := DeprecatedResult{
+		Objects:  map[types.Object]*IsDeprecated{},
+		Packages: map[*types.Package]*IsDeprecated{},
+	}
+
+	for _, fact := range pass.AllObjectFacts() {
+		out.Objects[fact.Object] = fact.Fact.(*IsDeprecated)
+	}
+	for _, fact := range pass.AllPackageFacts() {
+		out.Packages[fact.Package] = fact.Fact.(*IsDeprecated)
+	}
+
+	return out, nil
+}

vendor/honnef.co/go/tools/analysis/facts/directives.go

@@ -0,0 +1,20 @@
+package facts
+
+import (
+	"reflect"
+
+	"golang.org/x/tools/go/analysis"
+	"honnef.co/go/tools/analysis/lint"
+)
+
+func directives(pass *analysis.Pass) (interface{}, error) {
+	return lint.ParseDirectives(pass.Files, pass.Fset), nil
+}
+
+var Directives = &analysis.Analyzer{
+	Name:             "directives",
+	Doc:              "extracts linter directives",
+	Run:              directives,
+	RunDespiteErrors: true,
+	ResultType:       reflect.TypeOf([]lint.Directive{}),
+}

vendor/honnef.co/go/tools/analysis/facts/generated.go

@@ -0,0 +1,97 @@
+package facts
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+	"os"
+	"reflect"
+	"strings"
+
+	"golang.org/x/tools/go/analysis"
+)
+
+type Generator int
+
+// A list of known generators we can detect
+const (
+	Unknown Generator = iota
+	Goyacc
+	Cgo
+	Stringer
+	ProtocGenGo
+)
+
+var (
+	// used by cgo before Go 1.11
+	oldCgo = []byte("// Created by cgo - DO NOT EDIT")
+	prefix = []byte("// Code generated ")
+	suffix = []byte(" DO NOT EDIT.")
+	nl     = []byte("\n")
+	crnl   = []byte("\r\n")
+)
+
+func isGenerated(path string) (Generator, bool) {
+	f, err := os.Open(path)
+	if err != nil {
+		return 0, false
+	}
+	defer f.Close()
+	br := bufio.NewReader(f)
+	for {
+		s, err := br.ReadBytes('\n')
+		if err != nil && err != io.EOF {
+			return 0, false
+		}
+		s = bytes.TrimSuffix(s, crnl)
+		s = bytes.TrimSuffix(s, nl)
+		if bytes.HasPrefix(s, prefix) && bytes.HasSuffix(s, suffix) {
+			if len(s)-len(suffix) < len(prefix) {
+				return Unknown, true
+			}
+
+			text := string(s[len(prefix) : len(s)-len(suffix)])
+			switch text {
+			case "by goyacc.":
+				return Goyacc, true
+			case "by cmd/cgo;":
+				return Cgo, true
+			case "by protoc-gen-go.":
+				return ProtocGenGo, true
+			}
+			if strings.HasPrefix(text, `by "stringer `) {
+				return Stringer, true
+			}
+			if strings.HasPrefix(text, `by goyacc `) {
+				return Goyacc, true
+			}
+
+			return Unknown, true
+		}
+		if bytes.Equal(s, oldCgo) {
+			return Cgo, true
+		}
+		if err == io.EOF {
+			break
+		}
+	}
+	return 0, false
+}
+
+var Generated = &analysis.Analyzer{
+	Name: "isgenerated",
+	Doc:  "annotate file names that have been code generated",
+	Run: func(pass *analysis.Pass) (interface{}, error) {
+		m := map[string]Generator{}
+		for _, f := range pass.Files {
+			path := pass.Fset.PositionFor(f.Pos(), false).Filename
+			g, ok := isGenerated(path)
+			if ok {
+				m[path] = g
+			}
+		}
+		return m, nil
+	},
+	RunDespiteErrors: true,
+	ResultType:       reflect.TypeOf(map[string]Generator{}),
+}

vendor/honnef.co/go/tools/analysis/facts/nilness/nilness.go

@@ -0,0 +1,251 @@
+package nilness
+
+import (
+	"fmt"
+	"go/token"
+	"go/types"
+	"reflect"
+
+	"honnef.co/go/tools/go/ir"
+	"honnef.co/go/tools/go/types/typeutil"
+	"honnef.co/go/tools/internal/passes/buildir"
+
+	"golang.org/x/tools/go/analysis"
+)
+
+// neverReturnsNilFact denotes that a function's return value will never
+// be nil (typed or untyped). The analysis errs on the side of false
+// negatives.
+type neverReturnsNilFact struct {
+	Rets []neverNilness
+}
+
+func (*neverReturnsNilFact) AFact() {}
+func (fact *neverReturnsNilFact) String() string {
+	return fmt.Sprintf("never returns nil: %v", fact.Rets)
+}
+
+type Result struct {
+	m map[*types.Func][]neverNilness
+}
+
+var Analysis = &analysis.Analyzer{
+	Name:       "nilness",
+	Doc:        "Annotates return values that will never be nil (typed or untyped)",
+	Run:        run,
+	Requires:   []*analysis.Analyzer{buildir.Analyzer},
+	FactTypes:  []analysis.Fact{(*neverReturnsNilFact)(nil)},
+	ResultType: reflect.TypeOf((*Result)(nil)),
+}
+
+// MayReturnNil reports whether the ret's return value of fn might be
+// a typed or untyped nil value. The value of ret is zero-based. When
+// globalOnly is true, the only possible nil values are global
+// variables.
+//
+// The analysis has false positives: MayReturnNil can incorrectly
+// report true, but never incorrectly reports false.
+func (r *Result) MayReturnNil(fn *types.Func, ret int) (yes bool, globalOnly bool) {
+	if !typeutil.IsPointerLike(fn.Type().(*types.Signature).Results().At(ret).Type()) {
+		return false, false
+	}
+	if len(r.m[fn]) == 0 {
+		return true, false
+	}
+
+	v := r.m[fn][ret]
+	return v != neverNil, v == onlyGlobal
+}
+
+func run(pass *analysis.Pass) (interface{}, error) {
+	seen := map[*ir.Function]struct{}{}
+	out := &Result{
+		m: map[*types.Func][]neverNilness{},
+	}
+	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
+		impl(pass, fn, seen)
+	}
+
+	for _, fact := range pass.AllObjectFacts() {
+		out.m[fact.Object.(*types.Func)] = fact.Fact.(*neverReturnsNilFact).Rets
+	}
+
+	return out, nil
+}
+
+type neverNilness uint8
+
+const (
+	neverNil   neverNilness = 1
+	onlyGlobal neverNilness = 2
+	nilly      neverNilness = 3
+)
+
+func (n neverNilness) String() string {
+	switch n {
+	case neverNil:
+		return "never"
+	case onlyGlobal:
+		return "global"
+	case nilly:
+		return "nil"
+	default:
+		return "BUG"
+	}
+}
+
+func impl(pass *analysis.Pass, fn *ir.Function, seenFns map[*ir.Function]struct{}) []neverNilness {
+	if fn.Object() == nil {
+		// TODO(dh): support closures
+		return nil
+	}
+	if fact := new(neverReturnsNilFact); pass.ImportObjectFact(fn.Object(), fact) {
+		return fact.Rets
+	}
+	if fn.Pkg != pass.ResultOf[buildir.Analyzer].(*buildir.IR).Pkg {
+		return nil
+	}
+	if fn.Blocks == nil {
+		return nil
+	}
+	if _, ok := seenFns[fn]; ok {
+		// break recursion
+		return nil
+	}
+
+	seenFns[fn] = struct{}{}
+
+	seen := map[ir.Value]struct{}{}
+
+	var mightReturnNil func(v ir.Value) neverNilness
+	mightReturnNil = func(v ir.Value) neverNilness {
+		if _, ok := seen[v]; ok {
+			// break cycle
+			return nilly
+		}
+		if !typeutil.IsPointerLike(v.Type()) {
+			return neverNil
+		}
+		seen[v] = struct{}{}
+		switch v := v.(type) {
+		case *ir.MakeInterface:
+			return mightReturnNil(v.X)
+		case *ir.Convert:
+			return mightReturnNil(v.X)
+		case *ir.SliceToArrayPointer:
+			if typeutil.CoreType(v.Type()).(*types.Pointer).Elem().Underlying().(*types.Array).Len() == 0 {
+				return mightReturnNil(v.X)
+			} else {
+				// converting a slice to an array pointer of length > 0 panics if the slice is nil
+				return neverNil
+			}
+		case *ir.Slice:
+			return mightReturnNil(v.X)
+		case *ir.Phi:
+			ret := neverNil
+			for _, e := range v.Edges {
+				if n := mightReturnNil(e); n > ret {
+					ret = n
+				}
+			}
+			return ret
+		case *ir.Extract:
+			switch d := v.Tuple.(type) {
+			case *ir.Call:
+				if callee := d.Call.StaticCallee(); callee != nil {
+					ret := impl(pass, callee, seenFns)
+					if len(ret) == 0 {
+						return nilly
+					}
+					return ret[v.Index]
+				} else {
+					return nilly
+				}
+			case *ir.TypeAssert, *ir.Next, *ir.Select, *ir.MapLookup, *ir.TypeSwitch, *ir.Recv, *ir.Sigma:
+				// we don't need to look at the Extract's index
+				// because we've already checked its type.
+				return nilly
+			default:
+				panic(fmt.Sprintf("internal error: unhandled type %T", d))
+			}
+		case *ir.Call:
+			if callee := v.Call.StaticCallee(); callee != nil {
+				ret := impl(pass, callee, seenFns)
+				if len(ret) == 0 {
+					return nilly
+				}
+				return ret[0]
+			} else {
+				return nilly
+			}
+		case *ir.BinOp, *ir.UnOp, *ir.Alloc, *ir.FieldAddr, *ir.IndexAddr, *ir.Global, *ir.MakeSlice, *ir.MakeClosure, *ir.Function, *ir.MakeMap, *ir.MakeChan:
+			return neverNil
+		case *ir.Sigma:
+			iff, ok := v.From.Control().(*ir.If)
+			if !ok {
+				return nilly
+			}
+			binop, ok := iff.Cond.(*ir.BinOp)
+			if !ok {
+				return nilly
+			}
+			isNil := func(v ir.Value) bool {
+				k, ok := v.(*ir.Const)
+				if !ok {
+					return false
+				}
+				return k.Value == nil
+			}
+			if binop.X == v.X && isNil(binop.Y) || binop.Y == v.X && isNil(binop.X) {
+				op := binop.Op
+				if v.From.Succs[0] != v.Block() {
+					// we're in the false branch, negate op
+					switch op {
+					case token.EQL:
+						op = token.NEQ
+					case token.NEQ:
+						op = token.EQL
+					default:
+						panic(fmt.Sprintf("internal error: unhandled token %v", op))
+					}
+				}
+				switch op {
+				case token.EQL:
+					return nilly
+				case token.NEQ:
+					return neverNil
+				default:
+					panic(fmt.Sprintf("internal error: unhandled token %v", op))
+				}
+			}
+			return nilly
+		case *ir.ChangeType:
+			return mightReturnNil(v.X)
+		case *ir.Load:
+			if _, ok := v.X.(*ir.Global); ok {
+				return onlyGlobal
+			}
+			return nilly
+		case *ir.AggregateConst:
+			return neverNil
+		case *ir.TypeAssert, *ir.ChangeInterface, *ir.Field, *ir.Const, *ir.GenericConst, *ir.Index, *ir.MapLookup, *ir.Parameter, *ir.Recv, *ir.TypeSwitch:
+			return nilly
+		default:
+			panic(fmt.Sprintf("internal error: unhandled type %T", v))
+		}
+	}
+	ret := fn.Exit.Control().(*ir.Return)
+	out := make([]neverNilness, len(ret.Results))
+	export := false
+	for i, v := range ret.Results {
+		v := mightReturnNil(v)
+		out[i] = v
+		if v != nilly && typeutil.IsPointerLike(fn.Signature.Results().At(i).Type()) {
+			export = true
+		}
+	}
+	if export {
+		pass.ExportObjectFact(fn.Object(), &neverReturnsNilFact{out})
+	}
+	return out
+}

vendor/honnef.co/go/tools/analysis/facts/purity.go

@@ -0,0 +1,178 @@
+package facts
+
+import (
+	"go/types"
+	"reflect"
+
+	"honnef.co/go/tools/go/ir"
+	"honnef.co/go/tools/go/ir/irutil"
+	"honnef.co/go/tools/internal/passes/buildir"
+
+	"golang.org/x/tools/go/analysis"
+)
+
+type IsPure struct{}
+
+func (*IsPure) AFact()           {}
+func (d *IsPure) String() string { return "is pure" }
+
+type PurityResult map[*types.Func]*IsPure
+
+var Purity = &analysis.Analyzer{
+	Name:       "fact_purity",
+	Doc:        "Mark pure functions",
+	Run:        purity,
+	Requires:   []*analysis.Analyzer{buildir.Analyzer},
+	FactTypes:  []analysis.Fact{(*IsPure)(nil)},
+	ResultType: reflect.TypeOf(PurityResult{}),
+}
+
+var pureStdlib = map[string]struct{}{
+	"errors.New":                      {},
+	"fmt.Errorf":                      {},
+	"fmt.Sprintf":                     {},
+	"fmt.Sprint":                      {},
+	"sort.Reverse":                    {},
+	"strings.Map":                     {},
+	"strings.Repeat":                  {},
+	"strings.Replace":                 {},
+	"strings.Title":                   {},
+	"strings.ToLower":                 {},
+	"strings.ToLowerSpecial":          {},
+	"strings.ToTitle":                 {},
+	"strings.ToTitleSpecial":          {},
+	"strings.ToUpper":                 {},
+	"strings.ToUpperSpecial":          {},
+	"strings.Trim":                    {},
+	"strings.TrimFunc":                {},
+	"strings.TrimLeft":                {},
+	"strings.TrimLeftFunc":            {},
+	"strings.TrimPrefix":              {},
+	"strings.TrimRight":               {},
+	"strings.TrimRightFunc":           {},
+	"strings.TrimSpace":               {},
+	"strings.TrimSuffix":              {},
+	"(*net/http.Request).WithContext": {},
+}
+
+func purity(pass *analysis.Pass) (interface{}, error) {
+	seen := map[*ir.Function]struct{}{}
+	irpkg := pass.ResultOf[buildir.Analyzer].(*buildir.IR).Pkg
+	var check func(fn *ir.Function) (ret bool)
+	check = func(fn *ir.Function) (ret bool) {
+		if fn.Object() == nil {
+			// TODO(dh): support closures
+			return false
+		}
+		if pass.ImportObjectFact(fn.Object(), new(IsPure)) {
+			return true
+		}
+		if fn.Pkg != irpkg {
+			// Function is in another package but wasn't marked as
+			// pure, ergo it isn't pure
+			return false
+		}
+		// Break recursion
+		if _, ok := seen[fn]; ok {
+			return false
+		}
+
+		seen[fn] = struct{}{}
+		defer func() {
+			if ret {
+				pass.ExportObjectFact(fn.Object(), &IsPure{})
+			}
+		}()
+
+		if irutil.IsStub(fn) {
+			return false
+		}
+
+		if _, ok := pureStdlib[fn.Object().(*types.Func).FullName()]; ok {
+			return true
+		}
+
+		if fn.Signature.Results().Len() == 0 {
+			// A function with no return values is empty or is doing some
+			// work we cannot see (for example because of build tags);
+			// don't consider it pure.
+			return false
+		}
+
+		for _, param := range fn.Params {
+			// TODO(dh): this may not be strictly correct. pure code
+			// can, to an extent, operate on non-basic types.
+			if _, ok := param.Type().Underlying().(*types.Basic); !ok {
+				return false
+			}
+		}
+
+		// Don't consider external functions pure.
+		if fn.Blocks == nil {
+			return false
+		}
+		checkCall := func(common *ir.CallCommon) bool {
+			if common.IsInvoke() {
+				return false
+			}
+			builtin, ok := common.Value.(*ir.Builtin)
+			if !ok {
+				if common.StaticCallee() != fn {
+					if common.StaticCallee() == nil {
+						return false
+					}
+					if !check(common.StaticCallee()) {
+						return false
+					}
+				}
+			} else {
+				switch builtin.Name() {
+				case "len", "cap":
+				default:
+					return false
+				}
+			}
+			return true
+		}
+		for _, b := range fn.Blocks {
+			for _, ins := range b.Instrs {
+				switch ins := ins.(type) {
+				case *ir.Call:
+					if !checkCall(ins.Common()) {
+						return false
+					}
+				case *ir.Defer:
+					if !checkCall(&ins.Call) {
+						return false
+					}
+				case *ir.Select:
+					return false
+				case *ir.Send:
+					return false
+				case *ir.Go:
+					return false
+				case *ir.Panic:
+					return false
+				case *ir.Store:
+					return false
+				case *ir.FieldAddr:
+					return false
+				case *ir.Alloc:
+					return false
+				case *ir.Load:
+					return false
+				}
+			}
+		}
+		return true
+	}
+	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
+		check(fn)
+	}
+
+	out := PurityResult{}
+	for _, fact := range pass.AllObjectFacts() {
+		out[fact.Object.(*types.Func)] = fact.Fact.(*IsPure)
+	}
+	return out, nil
+}

vendor/honnef.co/go/tools/analysis/facts/token.go

@@ -0,0 +1,24 @@
+package facts
+
+import (
+	"go/ast"
+	"go/token"
+	"reflect"
+
+	"golang.org/x/tools/go/analysis"
+)
+
+var TokenFile = &analysis.Analyzer{
+	Name: "tokenfileanalyzer",
+	Doc:  "creates a mapping of *token.File to *ast.File",
+	Run: func(pass *analysis.Pass) (interface{}, error) {
+		m := map[*token.File]*ast.File{}
+		for _, af := range pass.Files {
+			tf := pass.Fset.File(af.Pos())
+			m[tf] = af
+		}
+		return m, nil
+	},
+	RunDespiteErrors: true,
+	ResultType:       reflect.TypeOf(map[*token.File]*ast.File{}),
+}

vendor/honnef.co/go/tools/analysis/facts/typedness/typedness.go

@@ -0,0 +1,253 @@
+package typedness
+
+import (
+	"fmt"
+	"go/token"
+	"go/types"
+	"reflect"
+
+	"honnef.co/go/tools/go/ir"
+	"honnef.co/go/tools/go/ir/irutil"
+	"honnef.co/go/tools/internal/passes/buildir"
+
+	"golang.org/x/exp/typeparams"
+	"golang.org/x/tools/go/analysis"
+)
+
+// alwaysTypedFact denotes that a function's return value will never
+// be untyped nil. The analysis errs on the side of false negatives.
+type alwaysTypedFact struct {
+	Rets uint8
+}
+
+func (*alwaysTypedFact) AFact() {}
+func (fact *alwaysTypedFact) String() string {
+	return fmt.Sprintf("always typed: %08b", fact.Rets)
+}
+
+type Result struct {
+	m map[*types.Func]uint8
+}
+
+var Analysis = &analysis.Analyzer{
+	Name:       "typedness",
+	Doc:        "Annotates return values that are always typed values",
+	Run:        run,
+	Requires:   []*analysis.Analyzer{buildir.Analyzer},
+	FactTypes:  []analysis.Fact{(*alwaysTypedFact)(nil)},
+	ResultType: reflect.TypeOf((*Result)(nil)),
+}
+
+// MustReturnTyped reports whether the ret's return value of fn must
+// be a typed value, i.e. an interface value containing a concrete
+// type or trivially a concrete type. The value of ret is zero-based.
+//
+// The analysis has false negatives: MustReturnTyped may incorrectly
+// report false, but never incorrectly reports true.
+func (r *Result) MustReturnTyped(fn *types.Func, ret int) bool {
+	if _, ok := fn.Type().(*types.Signature).Results().At(ret).Type().Underlying().(*types.Interface); !ok {
+		return true
+	}
+	return (r.m[fn] & (1 << ret)) != 0
+}
+
+func run(pass *analysis.Pass) (interface{}, error) {
+	seen := map[*ir.Function]struct{}{}
+	out := &Result{
+		m: map[*types.Func]uint8{},
+	}
+	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
+		impl(pass, fn, seen)
+	}
+
+	for _, fact := range pass.AllObjectFacts() {
+		out.m[fact.Object.(*types.Func)] = fact.Fact.(*alwaysTypedFact).Rets
+	}
+
+	return out, nil
+}
+
+func impl(pass *analysis.Pass, fn *ir.Function, seenFns map[*ir.Function]struct{}) (out uint8) {
+	if fn.Signature.Results().Len() > 8 {
+		return 0
+	}
+	if fn.Object() == nil {
+		// TODO(dh): support closures
+		return 0
+	}
+	if fact := new(alwaysTypedFact); pass.ImportObjectFact(fn.Object(), fact) {
+		return fact.Rets
+	}
+	if fn.Pkg != pass.ResultOf[buildir.Analyzer].(*buildir.IR).Pkg {
+		return 0
+	}
+	if fn.Blocks == nil {
+		return 0
+	}
+	if irutil.IsStub(fn) {
+		return 0
+	}
+	if _, ok := seenFns[fn]; ok {
+		// break recursion
+		return 0
+	}
+
+	seenFns[fn] = struct{}{}
+	defer func() {
+		for i := 0; i < fn.Signature.Results().Len(); i++ {
+			if _, ok := fn.Signature.Results().At(i).Type().Underlying().(*types.Interface); !ok {
+				// we don't need facts to know that non-interface
+				// types can't be untyped nil. zeroing out those bits
+				// may result in all bits being zero, in which case we
+				// don't have to save any fact.
+				out &= ^(1 << i)
+			}
+		}
+		if out > 0 {
+			pass.ExportObjectFact(fn.Object(), &alwaysTypedFact{out})
+		}
+	}()
+
+	isUntypedNil := func(v ir.Value) bool {
+		k, ok := v.(*ir.Const)
+		if !ok {
+			return false
+		}
+		if _, ok := k.Type().Underlying().(*types.Interface); !ok {
+			return false
+		}
+		return k.Value == nil
+	}
+
+	var do func(v ir.Value, seen map[ir.Value]struct{}) bool
+	do = func(v ir.Value, seen map[ir.Value]struct{}) bool {
+		if _, ok := seen[v]; ok {
+			// break cycle
+			return false
+		}
+		seen[v] = struct{}{}
+		switch v := v.(type) {
+		case *ir.Const:
+			// can't be a typed nil, because then we'd be returning the
+			// result of MakeInterface.
+			return false
+		case *ir.ChangeInterface:
+			return do(v.X, seen)
+		case *ir.Extract:
+			call, ok := v.Tuple.(*ir.Call)
+			if !ok {
+				// We only care about extracts of function results. For
+				// everything else (e.g. channel receives and map
+				// lookups), we can either not deduce any information, or
+				// will see a MakeInterface.
+				return false
+			}
+			if callee := call.Call.StaticCallee(); callee != nil {
+				return impl(pass, callee, seenFns)&(1<<v.Index) != 0
+			} else {
+				// we don't know what function we're calling. no need
+				// to look at the signature, though. if it weren't an
+				// interface, we'd be seeing a MakeInterface
+				// instruction.
+				return false
+			}
+		case *ir.Call:
+			if callee := v.Call.StaticCallee(); callee != nil {
+				return impl(pass, callee, seenFns)&1 != 0
+			} else {
+				// we don't know what function we're calling. no need
+				// to look at the signature, though. if it weren't an
+				// interface, we'd be seeing a MakeInterface
+				// instruction.
+				return false
+			}
+		case *ir.Sigma:
+			iff, ok := v.From.Control().(*ir.If)
+			if !ok {
+				// give up
+				return false
+			}
+
+			binop, ok := iff.Cond.(*ir.BinOp)
+			if !ok {
+				// give up
+				return false
+			}
+
+			if (binop.X == v.X && isUntypedNil(binop.Y)) || (isUntypedNil(binop.X) && binop.Y == v.X) {
+				op := binop.Op
+				if v.From.Succs[0] != v.Block() {
+					// we're in the false branch, negate op
+					switch op {
+					case token.EQL:
+						op = token.NEQ
+					case token.NEQ:
+						op = token.EQL
+					default:
+						panic(fmt.Sprintf("internal error: unhandled token %v", op))
+					}
+				}
+
+				switch op {
+				case token.EQL:
+					// returned value equals untyped nil
+					return false
+				case token.NEQ:
+					// returned value does not equal untyped nil
+					return true
+				default:
+					panic(fmt.Sprintf("internal error: unhandled token %v", op))
+				}
+			}
+
+			// TODO(dh): handle comparison with typed nil
+
+			// give up
+			return false
+		case *ir.Phi:
+			for _, pv := range v.Edges {
+				if !do(pv, seen) {
+					return false
+				}
+			}
+			return true
+		case *ir.MakeInterface:
+			terms, err := typeparams.NormalTerms(v.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.
+				//
+				// There is no instruction that can create a guaranteed non-nil instance of a type parameter without
+				// type constraints, so we return false right away, without checking v.X's typedness.
+				return false
+			}
+			return true
+		case *ir.TypeAssert:
+			// type assertions fail for untyped nils. Either we have a
+			// single lhs and the type assertion succeeds or panics,
+			// or we have two lhs and we'll return Extract instead.
+			return true
+		case *ir.ChangeType:
+			// we'll only see interface->interface conversions, which
+			// don't tell us anything about the nilness.
+			return false
+		case *ir.MapLookup, *ir.Index, *ir.Recv, *ir.Parameter, *ir.Load, *ir.Field:
+			// All other instructions that tell us nothing about the
+			// typedness of interface values.
+			return false
+		default:
+			panic(fmt.Sprintf("internal error: unhandled type %T", v))
+		}
+	}
+
+	ret := fn.Exit.Control().(*ir.Return)
+	for i, v := range ret.Results {
+		typ := fn.Signature.Results().At(i).Type()
+		if _, ok := typ.Underlying().(*types.Interface); ok && !typeparams.IsTypeParam(typ) {
+			if do(v, map[ir.Value]struct{}{}) {
+				out |= 1 << i
+			}
+		}
+	}
+	return out
+}