cmd/compile/internal/gc: fix panic in Type Stringer

The following code:

func n() {(interface{int})}

generates:

3: interface contains embedded non-interface int
3: type %!v(PANIC=runtime error: invalid memory address or nil pointer dereference) is not an expression

It is because the corresponding symbol (Sym field in Type object)
is nil, resulting in a panic in typefmt.

Just skip the symbol if it is nil, so that the error message becomes:

3: interface contains embedded non-interface int
3: type interface { int } is not an expression

Fixes #11614

Change-Id: I219ae7eb01edca264fad1d4a1bd261d026294b00
Reviewed-on: https://go-review.googlesource.com/14015Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

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

@@ -579,9 +579,14 @@ func typefmt(t *Type, flag int) string {
 		buf.WriteString("interface {")
 		for t1 := t.Type; t1 != nil; t1 = t1.Down {
 			buf.WriteString(" ")
-			if exportname(t1.Sym.Name) {
+			switch {
+			case t1.Sym == nil:
+				// Check first that a symbol is defined for this type.
+				// Wrong interface definitions may have types lacking a symbol.
+				break
+			case exportname(t1.Sym.Name):
 				buf.WriteString(Sconv(t1.Sym, obj.FmtShort))
-			} else {
+			default:
 				buf.WriteString(Sconv(t1.Sym, obj.FmtUnsigned))
 			}
 			buf.WriteString(Tconv(t1.Type, obj.FmtShort))

test/fixedbugs/issue11614.go

+// errorcheck
+
+// Copyright 2015 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.
+
+// Test that incorrect expressions involving wrong anonymous interface
+// do not generate panics in Type Stringer.
+// Does not compile.
+
+package main
+
+type I interface {
+	int // ERROR "interface contains embedded non-interface int"
+}
+
+func n() {
+	(I) // ERROR "type I is not an expression"
+}
+
+func m() {
+	(interface{int}) // ERROR "interface contains embedded non-interface int" "type interface { int } is not an expression"
+}
+
+func main() {
+}