gc: Fail on implicit assigment to non-exported fields in copy and append.

Fixes #1387.

R=rsc
CC=golang-dev
https://golang.org/cl/5348046

src/cmd/gc/typecheck.c

@@ -1032,6 +1032,9 @@ reswitch:
 			yyerror("first argument to append must be slice; have %lT", t);
 			goto error;
 		}
+		if(!exportassignok(t->type, "append"))
+			goto error;
+
 		if(n->isddd) {
 			if(args->next == nil) {
 				yyerror("cannot use ... on first argument to append");
@@ -1098,6 +1101,8 @@ reswitch:
 			yyerror("arguments to copy have different element types: %lT and %lT", n->left->type, n->right->type);
 			goto error;
 		}
+		if(!exportassignok(n->left->type->type, "copy"))
+			goto error;
 		goto ret;
 
 	case OCONV:

test/fixedbugs/bug378.go

@@ -0,0 +1,27 @@
+// errchk $G $D/$F.go
+
+// Copyright 2011 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.
+
+// Issue 1387
+package foo
+
+import "bytes"
+
+func i() {
+	a := make([]bytes.Buffer, 1)
+	b := a[0] // ERROR "unexported field"
+}
+
+func f() {
+	a := make([]bytes.Buffer, 1)
+	a = append(a, a...) // ERROR "unexported field"
+}
+
+
+func g() {
+	a := make([]bytes.Buffer, 1)
+	b := make([]bytes.Buffer, 1)
+	copy(b, a)	// ERROR "unexported field"
+}