cmd/gc: reject methods on non-locals, even if symbol exists

Fixes #5089.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7767044

src/cmd/gc/dcl.c

@@ -1380,6 +1380,12 @@ addmethod(Sym *sf, Type *t, int local, int nointerface)
 		}
 	}
 
+	if(local && !pa->local) {
+		// defining method on non-local type.
+		yyerror("cannot define new methods on non-local type %T", pa);
+		return;
+	}
+
 	n = nod(ODCLFIELD, newname(sf), N);
 	n->type = t;
 
@@ -1395,12 +1401,6 @@ addmethod(Sym *sf, Type *t, int local, int nointerface)
 		return;
 	}
 
-	if(local && !pa->local) {
-		// defining method on non-local type.
-		yyerror("cannot define new methods on non-local type %T", pa);
-		return;
-	}
-
 	f = structfield(n);
 	f->nointerface = nointerface;
 

test/fixedbugs/issue5089.go

+// errorcheck
+
+// Copyright 2013 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 5089: gc allows methods on non-locals if symbol already exists
+
+package p
+
+import "bufio"
+
+func (b *bufio.Reader) Buffered() int { // ERROR "non-local"
+	return -1
+}