reflect: fix DeepEqual for some cyclic corner cases

Fixes #15610.

Change-Id: Idbc8a9b328b92034d53b8009471678a166d5cf3f
Reviewed-on: https://go-review.googlesource.com/31588
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>

src/reflect/all_test.go

@@ -648,6 +648,20 @@ var (
 
 type self struct{}
 
+type Loop *Loop
+type Loopy interface{}
+
+var loop1, loop2 Loop
+var loopy1, loopy2 Loopy
+
+func init() {
+	loop1 = &loop2
+	loop2 = &loop1
+
+	loopy1 = &loopy2
+	loopy2 = &loopy1
+}
+
 var deepEqualTests = []DeepEqualTest{
 	// Equalities
 	{nil, nil, true},
@@ -706,6 +720,12 @@ var deepEqualTests = []DeepEqualTest{
 	{&[3]interface{}{1, 2, 4}, &[3]interface{}{1, 2, "s"}, false},
 	{Basic{1, 0.5}, NotBasic{1, 0.5}, false},
 	{map[uint]string{1: "one", 2: "two"}, map[int]string{2: "two", 1: "one"}, false},
+
+	// Possible loops.
+	{&loop1, &loop1, true},
+	{&loop1, &loop2, true},
+	{&loopy1, &loopy1, true},
+	{&loopy1, &loopy2, true},
 }
 
 func TestDeepEqual(t *testing.T) {

src/reflect/deepequal.go

@@ -30,9 +30,13 @@ func deepValueEqual(v1, v2 Value, visited map[visit]bool, depth int) bool {
 	}
 
 	// if depth > 10 { panic("deepValueEqual") }	// for debugging
+
+	// We want to avoid putting more in the visited map than we need to.
+	// For any possible reference cycle that might be encountered,
+	// hard(t) needs to return true for at least one of the types in the cycle.
 	hard := func(k Kind) bool {
 		switch k {
-		case Array, Map, Slice, Struct:
+		case Map, Slice, Ptr, Interface:
 			return true
 		}
 		return false