cmd/cgo: enforce typing of 0-sized types

cgo represents all 0-sized and unsized types internally as [0]byte. This means that pointers to incomplete types would be interchangable, even if given a name by typedef.

Fixes #7409.

LGTM=iant
R=golang-codereviews, bradfitz, iant
CC=golang-codereviews
https://golang.org/cl/76450043

misc/cgo/errors/err3.go

+// Copyright 2014 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
+
+/*
+typedef struct foo foo_t;
+typedef struct bar bar_t;
+
+foo_t *foop;
+*/
+import "C"
+
+func main() {
+	x := (*C.bar_t)(nil)
+	C.foop = x // ERROR HERE
+}

misc/cgo/errors/test.bash

@@ -26,6 +26,7 @@ check() {
 
 check err1.go
 check err2.go
+check err3.go
 
 rm -rf errs _obj
 exit 0

src/cmd/cgo/gcc.go

@@ -1327,9 +1327,12 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 		// be correct, so calling dtype.Size again will produce the correct value.
 		t.Size = dtype.Size()
 		if t.Size < 0 {
-			// Unsized types are [0]byte
+			// Unsized types are [0]byte, unless they're typedefs of other types.
+			// if so, use the name of the typedef for the go name.
 			t.Size = 0
-			t.Go = c.Opaque(0)
+			if _, ok := dtype.(*dwarf.TypedefType); !ok {
+				t.Go = c.Opaque(0)
+			}
 			if t.C.Empty() {
 				t.C.Set("void")
 			}