cmd/compile: confusing error if composite literal field is a method

When looking for the field specified in a composite literal, check that
the specified name is actually a field and not a method.

Fixes #29855.

Change-Id: Id77666e846f925907b1eec64213b1d25af8a2466
Reviewed-on: https://go-review.googlesource.com/c/158938
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/reflect.go

@@ -375,7 +375,7 @@ func methods(t *types.Type) []*Sig {
 	// generating code if necessary.
 	var ms []*Sig
 	for _, f := range mt.AllMethods().Slice() {
-		if f.Type.Etype != TFUNC || f.Type.Recv() == nil {
+		if !f.IsMethod() {
 			Fatalf("non-method on %v method %v %v\n", mt, f.Sym, f)
 		}
 		if f.Type.Recv() == nil {

src/cmd/compile/internal/gc/subr.go

@@ -1180,7 +1180,7 @@ func lookdot0(s *types.Sym, t *types.Type, save **types.Field, ignorecase bool)
 	c := 0
 	if u.IsStruct() || u.IsInterface() {
 		for _, f := range u.Fields().Slice() {
-			if f.Sym == s || (ignorecase && f.Type.Etype == TFUNC && f.Type.Recv() != nil && strings.EqualFold(f.Sym.Name, s.Name)) {
+			if f.Sym == s || (ignorecase && f.IsMethod() && strings.EqualFold(f.Sym.Name, s.Name)) {
 				if save != nil {
 					*save = f
 				}
@@ -1420,7 +1420,7 @@ func expandmeth(t *types.Type) {
 		}
 
 		// dotpath may have dug out arbitrary fields, we only want methods.
-		if f.Type.Etype != TFUNC || f.Type.Recv() == nil {
+		if !f.IsMethod() {
 			continue
 		}
 
@@ -1631,7 +1631,7 @@ func ifacelookdot(s *types.Sym, t *types.Type, ignorecase bool) (m *types.Field,
 		}
 	}
 
-	if m.Type.Etype != TFUNC || m.Type.Recv() == nil {
+	if !m.IsMethod() {
 		yyerror("%v.%v is a field, not a method", t, s)
 		return nil, followptr
 	}

src/cmd/compile/internal/gc/typecheck.go

@@ -3250,8 +3250,9 @@ func typecheckcomplit(n *Node) (res *Node) {
 						}
 						continue
 					}
-					p, _ := dotpath(l.Sym, t, nil, true)
-					if p == nil {
+					var f *types.Field
+					p, _ := dotpath(l.Sym, t, &f, true)
+					if p == nil || f.IsMethod() {
 						yyerror("unknown field '%v' in struct literal of type %v", l.Sym, t)
 						continue
 					}

src/cmd/compile/internal/types/type.go

@@ -392,6 +392,11 @@ func (f *Field) End() int64 {
 	return f.Offset + f.Type.Width
 }
 
+// IsMethod reports whether f represents a method rather than a struct field.
+func (f *Field) IsMethod() bool {
+	return f.Type.Etype == TFUNC && f.Type.Recv() != nil
+}
+
 // Fields is a pointer to a slice of *Field.
 // This saves space in Types that do not have fields or methods
 // compared to a simple slice of *Field.

test/fixedbugs/issue29855.go

@@ -0,0 +1,17 @@
+// errorcheck
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+type T struct {
+	GlobalName string
+}
+
+var t = T{Name: "foo"} // ERROR "unknown field 'Name' in struct literal of type T"
+
+func (t T) Name() string {
+	return t.GlobalName
+}