gc: empty select

R=ken2
CC=golang-dev
https://golang.org/cl/1871057

src/cmd/gc/select.c

@@ -68,8 +68,6 @@ typecheckselect(Node *sel)
 		typechecklist(ncase->nbody, Etop);
 	}
 	sel->xoffset = count;
-	if(count == 0)
-		yyerror("empty select");
 	lineno = lno;
 }
 
@@ -91,7 +89,7 @@ walkselect(Node *sel)
 	typecheck(&r, Etop);
 	init = list(init, r);
 
-	if(sel->list == nil)
+	if(sel->list == nil && sel->xoffset != 0)
 		fatal("double walkselect");	// already rewrote
 
 	// register cases

src/pkg/runtime/chan.c

@@ -631,9 +631,11 @@ void
 		printf("select: sel=%p\n", sel);
 
 	if(sel->ncase < 2) {
-		if(sel->ncase < 1)
-			throw("select: no cases");
-		// make special case of one.
+		if(sel->ncase < 1) {
+			g->status = Gwaiting;	// forever
+			gosched();
+		}
+		// TODO: make special case of one.
 	}
 
 	// select a (relative) prime

test/chan/select3.go

@@ -112,13 +112,9 @@ func main() {
 		<-ch
 	})
 
-	// TODO(gri) remove this if once 6g accepts empty selects
-	enabled := false
-	if enabled {
 	// empty selects always block
 	testBlock(always, func() {
 		select {
-			case <-make(chan int): // remove this once 6g accepts empty selects
 		}
 	})
 
@@ -143,7 +139,6 @@ func main() {
 			unreachable()
 		}
 	})
-	}
 
 	// selects with non-ready non-nil channels always block
 	testBlock(always, func() {