gc: bug278

Fixes #804.

R=ken2
CC=golang-dev
https://golang.org/cl/1224045

src/cmd/gc/typecheck.c

@@ -2074,6 +2074,9 @@ islvalue(Node *n)
 {
 	switch(n->op) {
 	case OINDEX:
+		if(isfixedarray(n->left->type))
+			return islvalue(n->left);
+		// fall through
 	case OIND:
 	case ODOTPTR:
 		return 1;

test/fixedbugs/bug278.go

@@ -0,0 +1,23 @@
+// errchk $G $D/$F.go
+
+// Copyright 2010 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.
+
+// This is a test case for issue 804.
+
+package main
+
+func f() [10]int {
+	return [10]int{}
+}
+
+var m map[int][10]int
+
+func main() {
+	f()[1] = 2	// ERROR "cannot"
+	f()[2:3][0] = 4	// ERROR "cannot"
+	var x = "abc"
+	x[2] = 3	// ERROR "cannot"
+	m[0][5] = 6  // ERROR "cannot"
+}