cmd/gc: disallow selectors to the blank identifier

Fixes #4941.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7415051

src/cmd/gc/typecheck.c

@@ -761,6 +761,10 @@ reswitch:
 			n->op = ODOTPTR;
 			checkwidth(t);
 		}
+		if(isblank(n->right)) {
+			yyerror("cannot refer to blank field or method");
+			goto error;
+		}
 		if(!lookdot(n, t, 0)) {
 			if(lookdot(n, t, 1))
 				yyerror("%N undefined (cannot refer to unexported field or method %S)", n, n->right->sym);

test/blank1.go

@@ -9,8 +9,13 @@
 
 package _	// ERROR "invalid package name _"
 
+var t struct {
+	_ int
+}
+
 func main() {
 	_()	// ERROR "cannot use _ as value"
 	x := _+1	// ERROR "cannot use _ as value"
 	_ = x
+	_ = t._ // ERROR "cannot refer to blank field"
 }