cmd/gc: disallow switch _ := v.(type)

Fixes #2827.

R=ken2
CC=golang-dev
https://golang.org/cl/5638045

src/cmd/gc/go.y

@@ -423,7 +423,7 @@ simple_stmt:
 				yyerror("expr.(type) must be alone in list");
 			if($1->next != nil)
 				yyerror("argument count mismatch: %d = %d", count($1), 1);
-			else if($1->n->op != ONAME && $1->n->op != OTYPE && $1->n->op != ONONAME)
+			else if(($1->n->op != ONAME && $1->n->op != OTYPE && $1->n->op != ONONAME) || isblank($1->n))
 				yyerror("invalid variable name %N in type switch", $1->n);
 			else
 				$$->left = dclname($1->n->sym);  // it's a colas, so must not re-use an oldname.

src/cmd/gc/y.tab.c

@@ -2714,7 +2714,7 @@ yyreduce:
 				yyerror("expr.(type) must be alone in list");
 			if((yyvsp[(1) - (3)].list)->next != nil)
 				yyerror("argument count mismatch: %d = %d", count((yyvsp[(1) - (3)].list)), 1);
-			else if((yyvsp[(1) - (3)].list)->n->op != ONAME && (yyvsp[(1) - (3)].list)->n->op != OTYPE && (yyvsp[(1) - (3)].list)->n->op != ONONAME)
+			else if(((yyvsp[(1) - (3)].list)->n->op != ONAME && (yyvsp[(1) - (3)].list)->n->op != OTYPE && (yyvsp[(1) - (3)].list)->n->op != ONONAME) || isblank((yyvsp[(1) - (3)].list)->n))
 				yyerror("invalid variable name %N in type switch", (yyvsp[(1) - (3)].list)->n);
 			else
 				(yyval.node)->left = dclname((yyvsp[(1) - (3)].list)->n->sym);  // it's a colas, so must not re-use an oldname.

test/typeswitch3.go

@@ -30,6 +30,10 @@ func main(){
 	switch r.(type) {
 	case io.Writer:
 	}
+	
+	// Issue 2827.
+	switch _ := r.(type) {  // ERROR "invalid variable name _"
+	}
 }