cmd/compile: a dot expression can not be a struct literal key

Passes toolstash -cmp.

Fixes #15311.

Change-Id: I1d67f5c9de38e899ab2d6c8986fabd6f197df23a
Reviewed-on: https://go-review.googlesource.com/22162Reviewed-by: David Crawshaw <crawshaw@golang.org>

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

@@ -3099,7 +3099,12 @@ func typecheckcomplit(n *Node) *Node {
 				}
 
 				s := l.Left.Sym
-				if s == nil {
+
+				// An OXDOT uses the Sym field to hold
+				// the field to the right of the dot,
+				// so s will be non-nil, but an OXDOT
+				// is never a valid struct literal key.
+				if s == nil || l.Left.Op == OXDOT {
 					Yyerror("invalid field name %v in struct initializer", l.Left)
 					l.Right = typecheck(l.Right, Erv)
 					continue

test/fixedbugs/issue15311.go

+// errorcheck
+
+// Copyright 2016 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.
+
+// The compiler was failing to correctly report an error when a dot
+// expression was used a struct literal key.
+
+package p
+
+type T struct {
+        toInt    map[string]int
+        toString map[int]string
+}
+
+var t = T{
+        foo.toInt:    make(map[string]int), // ERROR "field name"
+        bar.toString: make(map[int]string), // ERROR "field name"
+}