cmd/gc: blank methods are not permitted in interface types

Fixes #6606.

LGTM=rsc
R=rsc
CC=golang-codereviews, gri
https://golang.org/cl/156210044

src/cmd/gc/dcl.c

@@ -558,6 +558,9 @@ ifacedcl(Node *n)
 	if(n->op != ODCLFIELD || n->right == N)
 		fatal("ifacedcl");
 
+	if(isblank(n->left))
+		yyerror("methods must have a unique non-blank name");
+
 	dclcontext = PPARAM;
 	markdcl();
 	funcdepth++;

test/interface/explicit.go

@@ -83,12 +83,12 @@ var m4 = M(jj) // ERROR "invalid|wrong type for M method"
 
 
 type B1 interface {
-	_()
+	_() // ERROR "methods must have a unique non-blank name"
 }
 
 type B2 interface {
 	M()
-	_()
+	_() // ERROR "methods must have a unique non-blank name"
 }
 
 type T2 struct{}

test/interface/fail.go

@@ -14,7 +14,6 @@ type I interface {
 
 func main() {
 	shouldPanic(p1)
-	shouldPanic(p2)
 }
 
 func p1() {
@@ -30,19 +29,6 @@ type S struct{}
 
 func (s *S) _() {}
 
-type B interface {
-	_()
-}
-
-func p2() {
-	var s *S
-	var b B
-	var e interface{}
-	e = s
-	b = e.(B)
-	_ = b
-}
-
 func shouldPanic(f func()) {
 	defer func() {
 		if recover() == nil {