gc: diagnose field+method of same name

Fixes #2828.

R=ken2
CC=golang-dev
https://golang.org/cl/5653065

src/cmd/gc/dcl.c

@@ -1303,6 +1303,14 @@ addmethod(Sym *sf, Type *t, int local)
 	}
 
 	pa = f;
+	if(pa->etype == TSTRUCT) {
+		for(f=pa->type; f; f=f->down) {
+			if(f->sym == sf) {
+				yyerror("type %T has both field and method named %S", pa, sf);
+				return;
+			}
+		}
+	}
 
 	n = nod(ODCLFIELD, newname(sf), N);
 	n->type = t;

test/fixedbugs/bug416.go

+// errchk $G $D/$F.go
+
+// Copyright 2012 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 p
+
+type T struct {
+	X int
+}
+
+func (t *T) X() {} // ERROR "type T has both field and method named X"