gc: Don't pollute the xmethod list with non-methods.

Fixes #2355.

I have a test, but not sure if it's worth adding.  Instead i've made
the patching-over in reflect.c methods more fatal and more descriptive.

R=rsc
CC=golang-dev
https://golang.org/cl/5302082

src/cmd/gc/reflect.c

@@ -158,10 +158,13 @@ methods(Type *t)
 	// generating code if necessary.
 	a = nil;
 	for(f=mt->xmethod; f; f=f->down) {
-		if(f->type->etype != TFUNC)
-			continue;
 		if(f->etype != TFIELD)
-			fatal("methods: not field");
+			fatal("methods: not field %T", f);
+		if (f->type->etype != TFUNC || f->type->thistuple == 0)
+			fatal("non-method on %T method %S %T\n", mt, f->sym, f);
+		if (!getthisx(f->type)->type)
+			fatal("receiver with no type on %T method %S %T\n", mt, f->sym, f);
+
 		method = f->sym;
 		if(method == nil)
 			continue;

src/cmd/gc/subr.c

@@ -2178,8 +2178,11 @@ expandmeth(Sym *s, Type *t)
 			if(c == 0)
 				continue;
 			if(c == 1) {
-				sl->good = 1;
-				sl->field = f;
+				// addot1 may have dug out arbitrary fields, we only want methods.
+				if(f->type->etype == TFUNC && f->type->thistuple > 0) {
+					sl->good = 1;
+					sl->field = f;
+				}
 			}
 			break;
 		}

test/fixedbugs/bug372.go

+// $G $D/$F.go && $L $F.$A && ./$A.out || echo BUG: bug372
+
+// 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 2355
+package main
+
+type T struct {}
+func (T) m() string { return "T" }
+
+type TT struct {
+	T
+	m func() string
+}
+
+
+func ff() string { return "ff" }
+
+func main() {
+	var tt TT
+	tt.m = ff
+
+	if tt.m() != "ff" {
+		println(tt.m(), "!= \"ff\"")
+	}
+}