cmd/compile: reject slice/map/func comparisons against converted nil

Fixes #13480.

Change-Id: Icbf4f83e965e84f7020f56c3f346193f8b91e7bf
Reviewed-on: https://go-review.googlesource.com/17461Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -760,17 +760,11 @@ func treecopy(n *Node, lineno int32) *Node {
 	return m
 }
 
+// isnil reports whether n represents the universal untyped zero value "nil".
 func isnil(n *Node) bool {
-	if n == nil {
-		return false
-	}
-	if n.Op != OLITERAL {
-		return false
-	}
-	if n.Val().Ctype() != CTNIL {
-		return false
-	}
-	return true
+	// Check n.Orig because constant propagation may produce typed nil constants,
+	// which don't exist in the Go spec.
+	return Isconst(n.Orig, CTNIL)
 }
 
 func isptrto(t *Type, et EType) bool {

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

@@ -137,7 +137,7 @@ func typecheckswitch(n *Node) {
 						} else {
 							Yyerror("invalid case %v in switch (mismatched types %v and bool)", ll.N, ll.N.Type)
 						}
-					case nilonly != "" && !Isconst(ll.N, CTNIL):
+					case nilonly != "" && !isnil(ll.N):
 						Yyerror("invalid case %v in switch (can only compare %s %v to nil)", ll.N, nilonly, n.Left)
 					}
 

test/fixedbugs/issue13480.go

+// errorcheck
+
+// Copyright 2015 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.
+
+// Verify that comparisons of slice/map/func values against converted nil
+// values are properly rejected.
+
+package p
+
+func bug() {
+	type S []byte
+	type M map[int]int
+	type F func()
+
+	var s S
+	var m M
+	var f F
+
+	_ = s == S(nil) // ERROR "compare.*to nil"
+	_ = S(nil) == s // ERROR "compare.*to nil"
+	switch s {
+	case S(nil): // ERROR "compare.*to nil"
+	}
+
+	_ = m == M(nil) // ERROR "compare.*to nil"
+	_ = M(nil) == m // ERROR "compare.*to nil"
+	switch m {
+	case M(nil): // ERROR "compare.*to nil"
+	}
+
+	_ = f == F(nil) // ERROR "compare.*to nil"
+	_ = F(nil) == f // ERROR "compare.*to nil"
+	switch f {
+	case F(nil): // ERROR "compare.*to nil"
+	}
+}