cmd/compile: disallow multiple nil cases in a type switch

Fixes #15898.

Change-Id: I66e2ad21f283563c7142aa820f0354711d964768
Reviewed-on: https://go-review.googlesource.com/23573
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -104,7 +104,7 @@ func typecheckswitch(n *Node) {
 
 	n.Type = t
 
-	var def *Node
+	var def, niltype *Node
 	for _, ncase := range n.List.Slice() {
 		setlineno(n)
 		if ncase.List.Len() == 0 {
@@ -150,6 +150,12 @@ func typecheckswitch(n *Node) {
 					var ptr int
 					switch {
 					case n1.Op == OLITERAL && n1.Type.IsKind(TNIL):
+						// case nil:
+						if niltype != nil {
+							Yyerror("multiple nil cases in type switch (first at %v)", niltype.Line())
+						} else {
+							niltype = ncase
+						}
 					case n1.Op != OTYPE && n1.Type != nil: // should this be ||?
 						Yyerror("%v is not a type", Nconv(n1, FmtLong))
 						// reset to original type

test/fixedbugs/issue15898.go

+// errorcheck
+
+// Copyright 2016 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(e interface{}) {
+	switch e.(type) {
+	case nil, nil: // ERROR "multiple nil cases in type switch"
+	}
+
+	switch e.(type) {
+	case nil:
+	case nil: // ERROR "multiple nil cases in type switch"
+	}
+}