cmd/compile: reject type switch with guarded declaration and no cases

Fixes #23116.

Change-Id: I5db5c5c39bbb50148ffa18c9393b045f255f80a3
Reviewed-on: https://go-review.googlesource.com/100459
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/swt.go

@@ -70,6 +70,12 @@ func typecheckswitch(n *Node) {
 		if t != nil && !t.IsInterface() {
 			yyerrorl(n.Pos, "cannot type switch on non-interface value %L", n.Left.Right)
 		}
+		if v := n.Left.Left; v != nil && !isblank(v) && n.List.Len() == 0 {
+			// We don't actually declare the type switch's guarded
+			// declaration itself. So if there are no cases, we
+			// won't notice that it went unused.
+			yyerrorl(v.Pos, "%v declared and not used", v.Sym)
+		}
 	} else {
 		// expression switch
 		top = Erv

test/fixedbugs/issue23116.go

+// errorcheck
+
+// Copyright 2018 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.(type) {
+	}
+
+	switch t := x.(type) { // ERROR "declared and not used"
+	}
+}