cmd/gc: disallow call of *T method using **T variable

This brings cmd/gc in line with the spec on this question.
It might break existing code, but that code was not conformant
with the spec.

Credit to Rémy for finding the broken code.

Fixes #6366.

LGTM=r
R=golang-codereviews, r
CC=adonovan, golang-codereviews, gri
https://golang.org/cl/129550043

src/cmd/gc/typecheck.c

@@ -2127,13 +2127,16 @@ lookdot(Node *n, Type *t, int dostrcmp)
 				n->left = nod(OADDR, n->left, N);
 				n->left->implicit = 1;
 				typecheck(&n->left, Etype|Erv);
-			} else if(tt->etype == tptr && eqtype(tt->type, rcvr)) {
+			} else if(tt->etype == tptr && rcvr->etype != tptr && eqtype(tt->type, rcvr)) {
 				n->left = nod(OIND, n->left, N);
 				n->left->implicit = 1;
 				typecheck(&n->left, Etype|Erv);
-			} else if(tt->etype == tptr && tt->type->etype == tptr && eqtype(derefall(tt), rcvr)) {
+			} else if(tt->etype == tptr && tt->type->etype == tptr && eqtype(derefall(tt), derefall(rcvr))) {
 				yyerror("calling method %N with receiver %lN requires explicit dereference", n->right, n->left);
 				while(tt->etype == tptr) {
+					// Stop one level early for method with pointer receiver.
+					if(rcvr->etype == tptr && tt->type->etype != tptr)
+						break;
 					n->left = nod(OIND, n->left, N);
 					n->left->implicit = 1;
 					typecheck(&n->left, Etype|Erv);

test/fixedbugs/bug371.go

@@ -8,10 +8,10 @@
 
 package main
 
-type T struct {}
+type T struct{}
 
 func (t *T) pm() {}
-func (t T) m() {}
+func (t T) m()   {}
 
 func main() {
 	p := &T{}
@@ -20,5 +20,5 @@ func main() {
 
 	q := &p
 	q.m()  // ERROR "requires explicit dereference"
-	q.pm()
+	q.pm() // ERROR "requires explicit dereference"
 }