cmd/gc: do not accept (**T).Method expressions.

The typechecking code was doing an extra, unnecessary
indirection.

Fixes #4458.

R=golang-dev, daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/6998051

src/cmd/gc/typecheck.c

@@ -1781,7 +1781,7 @@ lookdot1(Node *errnode, Sym *s, Type *t, Type *f, int dostrcmp)
 static int
 looktypedot(Node *n, Type *t, int dostrcmp)
 {
-	Type *f1, *f2, *tt;
+	Type *f1, *f2;
 	Sym *s;
 	
 	s = n->right->sym;
@@ -1798,11 +1798,9 @@ looktypedot(Node *n, Type *t, int dostrcmp)
 		return 1;
 	}
 
-	tt = t;
-	if(t->sym == S && isptr[t->etype])
-		tt = t->type;
-
-	f2 = methtype(tt, 0);
+	// Find the base type: methtype will fail if t
+	// is not of the form T or *T.
+	f2 = methtype(t, 0);
 	if(f2 == T)
 		return 0;
 

test/fixedbugs/issue4458.go

@@ -0,0 +1,20 @@
+// errorcheck
+
+// 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.
+
+// Issue 4458: gc accepts invalid method expressions
+// like (**T).Method.
+
+package main
+
+type T struct{}
+
+func (T) foo() {}
+
+func main() {
+	av := T{}
+	pav := &av
+	(**T).foo(&pav) // ERROR "no method foo"
+}