cmd/gc: typecheck type switch variables

Type switch variables was not typechecked.
Previously it lead only to a minor consequence:

switch unsafe.Sizeof = x.(type) {

generated an inconsistent error message.
But capturing by value functionality now requries typechecking of all ONAMEs.

Fixes #9731

Change-Id: If037883cba53d85028fb97b1328696091b3b7ddd
Reviewed-on: https://go-review.googlesource.com/3600
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gc/swt.c

@@ -939,6 +939,8 @@ typecheckswitch(Node *n)
 					// multiple entry type switch or default
 					nvar->ntype = typenod(n->type);
 				}
+				typecheck(&nvar, Erv | Easgn);
+				ncase->nname = nvar;
 			}
 		}
 		typechecklist(ncase->nbody, Etop);

test/fixedbugs/issue9731.go

@@ -0,0 +1,21 @@
+// compile
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f(x interface{}) {
+	switch x := x.(type) {
+	case int:
+		func() {
+			_ = x
+		}()
+	case map[int]int:
+		func() {
+			for range x {
+			}
+		}()
+	}
+}