reflect: panic when Value.Equal using two non-comparable values

Assuming the two values are valid and non-comparable, Equal should panic.

	x := reflect.ValueOf([]int{1, 2, 3})
	x.Equal(x) // can not report false, should panic

Assuming one of them is non-comparable and the other is invalid, it should
always report false.

	x := reflect.ValueOf([]int{1, 2, 3})
	y := reflect.ValueOf(nil)
	x.Equal(y) // should report false

For #46746.

Change-Id: Ifecd77ca0b3de3019fae2be39048f9277831676c
Reviewed-on: https://go-review.googlesource.com/c/go/+/440037
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/reflect/all_test.go

@@ -8244,16 +8244,6 @@ var valueEqualTests = []ValueEqualTest{
 		true,
 		true, false,
 	},
-	{
-		&equalSlice, []int{1},
-		false,
-		true, false,
-	},
-	{
-		map[int]int{}, map[int]int{},
-		false,
-		false, false,
-	},
 	{
 		(chan int)(nil), nil,
 		false,
@@ -8289,11 +8279,6 @@ var valueEqualTests = []ValueEqualTest{
 		true,
 		false, false,
 	},
-	{
-		&mapInterface, &mapInterface,
-		false,
-		true, true,
-	},
 }
 
 func TestValue_Equal(t *testing.T) {
@@ -8324,6 +8309,41 @@ func TestValue_Equal(t *testing.T) {
 	}
 }
 
+func TestValue_EqualNonComparable(t *testing.T) {
+	var invalid = Value{} // ValueOf(nil)
+	var values = []Value{
+		// Value of slice is non-comparable.
+		ValueOf([]int(nil)),
+		ValueOf(([]int{})),
+
+		// Value of map is non-comparable.
+		ValueOf(map[int]int(nil)),
+		ValueOf((map[int]int{})),
+
+		// Value of func is non-comparable.
+		ValueOf(((func())(nil))),
+		ValueOf(func() {}),
+
+		// Value of struct is non-comparable because of non-comparable elements.
+		ValueOf((NonComparableStruct{})),
+
+		// Value of array is non-comparable because of non-comparable elements.
+		ValueOf([0]map[int]int{}),
+		ValueOf([0]func(){}),
+		ValueOf(([1]struct{ I interface{} }{{[]int{}}})),
+		ValueOf(([1]interface{}{[1]interface{}{map[int]int{}}})),
+	}
+	for _, value := range values {
+		// Panic when reflect.Value.Equal using two valid non-comparable values.
+		shouldPanic("reflect.Value.Equal using two non-comparable values", func() { value.Equal(value) })
+
+		// If one is non-comparable and the other is invalid, the expected result is always false.
+		if r := value.Equal(invalid); r != false {
+			t.Errorf("%s == invalid got %t, want false", value.Type(), r)
+		}
+	}
+}
+
 func TestInitFuncTypes(t *testing.T) {
 	n := 100
 	var wg sync.WaitGroup

src/reflect/value.go

@@ -3320,7 +3320,8 @@ func (v Value) Comparable() bool {
 }
 
 // Equal reports true if v is equal to u.
-// For valid values, if either v or u is non-comparable, Equal returns false.
+// For valid values, if one of them is non-comparable, and the other is comparable,
+// Equal reports false; if v and u are both non-comparable, Equal will panic.
 func (v Value) Equal(u Value) bool {
 	if !v.IsValid() || !u.IsValid() {
 		return v.IsValid() == u.IsValid()
@@ -3334,7 +3335,7 @@ func (v Value) Equal(u Value) bool {
 		return v.Elem().Equal(u.Elem())
 	}
 
-	return false
+	panic("reflect.Value.Equal using two non-comparable values")
 }
 
 // convertOp returns the function to convert a value of type src