more precise error message

package main

func main() {
       var x interface {} = 42;
       switch x := x.(type) {
       case int:
       case foo:
       }
}

before:
x.go:7: non-type case in type switch
x.go:7: inappropriate case for a type switch

now:
x.go:7: foo: undefined

R=ken
OCL=31221
CL=31221

src/cmd/gc/go.y

@@ -468,6 +468,7 @@ case:
 		// done in casebody()
 		poptodcl();
 		if(typeswvar != N && typeswvar->right != N) {
+			int e;
 			if($2->op == OLITERAL && $2->val.ctype == CTNIL) {
 				// this version in type switch case nil
 				$$ = nod(OTYPESW, N, N);
@@ -481,7 +482,16 @@ case:
 				addtotop($$);
 				break;
 			}
-			yyerror("non-type case in type switch");
+			e = nerrors;
+			gettype($2, N);
+			// maybe gettype found problems that keep
+			// e from being valid even outside a type switch.
+			// only complain if gettype didn't print new errors.
+			if(nerrors == e)
+				yyerror("non-type case in type switch");
+			$$ = nod(OXCASE, N, N);
+			$$->diag = 1;
+			break;
 		}
 		$$ = nod(OXCASE, $2, N);
 	}

src/cmd/gc/swt.c

@@ -326,7 +326,7 @@ loop:
 	if(n->op != OCASE)
 		fatal("walkcases: not case %O\n", n->op);
 
-	if(n->left != N) {
+	if(n->left != N && !n->diag) {
 		setlineno(n);
 		place = call(n->left, place, arg);
 	}