errors: fix Is panics if target is uncomparable

Fixes #31841

Change-Id: I3f068686154fd2fa5755b0df47b4eaa5c9a19107
Reviewed-on: https://go-review.googlesource.com/c/go/+/175260Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/errors/wrap.go

@@ -51,8 +51,10 @@ func Is(err, target error) bool {
 	if target == nil {
 		return err == target
 	}
+
+	isComparable := target == nil || reflectlite.TypeOf(target).Comparable()
 	for {
-		if err == target {
+		if isComparable && err == target {
 			return true
 		}
 		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {

src/errors/wrap_test.go

@@ -47,6 +47,12 @@ func TestIs(t *testing.T) {
 		{poser, errb, false},
 		{poser, erro, false},
 		{poser, errco, false},
+		{errorUncomparable{}, errorUncomparable{}, true},
+		{errorUncomparable{}, &errorUncomparable{}, false},
+		{&errorUncomparable{}, errorUncomparable{}, true},
+		{&errorUncomparable{}, &errorUncomparable{}, false},
+		{errorUncomparable{}, err1, false},
+		{&errorUncomparable{}, err1, false},
 	}
 	for _, tc := range testCases {
 		t.Run("", func(t *testing.T) {
@@ -260,3 +266,16 @@ type printer struct {
 }
 
 func (p *printer) Print(args ...interface{}) { fmt.Fprint(&p.buf, args...) }
+
+type errorUncomparable struct {
+	f []string
+}
+
+func (errorUncomparable) Error() string {
+	return "uncomparable error"
+}
+
+func (errorUncomparable) Is(target error) bool {
+	_, ok := target.(errorUncomparable)
+	return ok
+}