cmd/compile: fix error when typeswitching on untyped

Fixes #20185
Fixes #19977

Change-Id: I8434713d20616ff0c0bc8f33f017e2548a5cccfa
Reviewed-on: https://go-review.googlesource.com/42210
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -177,7 +177,14 @@ func typecheckswitch(n *Node) {
 			}
 		}
 
-		if top == Etype && n.Type != nil {
+		if n.Type == nil || n.Type.IsUntyped() {
+			// if the value we're switching on has no type or is untyped,
+			// we've already printed an error and don't need to continue
+			// typechecking the body
+			return
+		}
+
+		if top == Etype {
 			ll := ncase.List
 			if ncase.Rlist.Len() != 0 {
 				nvar := ncase.Rlist.First()

test/fixedbugs/issue19977.go

+// errorcheck
+
+// Copyright 2017 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.
+
+// Issue 19977: multiple error messages when type switching on an undefined
+
+package foo
+
+func Foo() {
+	switch x := a.(type) { // ERROR "undefined: a"
+	default:
+		_ = x
+	}
+}

test/fixedbugs/issue20185.go

+// errorcheck
+
+// Copyright 2017 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.
+
+// Issue 20185: type switching on untyped values (e.g. nil or consts)
+// caused an internal compiler error.
+
+package p
+
+func F() {
+	switch t := nil.(type) { // ERROR "cannot type switch on non-interface value nil"
+	default:
+		_ = t
+	}
+}
+
+const x = 1
+
+func G() {
+	switch t := x.(type) { // ERROR "cannot type switch on non-interface value x \(type untyped number\)"
+	default:
+	}
+}