reflect: empty slice/map is not DeepEqual to nil

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5373095

src/pkg/reflect/all_test.go

@@ -651,6 +651,14 @@ var deepEqualTests = []DeepEqualTest{
 	{nil, 1, false},
 	{1, nil, false},
 
+	// Nil vs empty: not the same.
+	{[]int{}, []int(nil), false},
+	{[]int{}, []int{}, true},
+	{[]int(nil), []int(nil), true},
+	{map[int]int{}, map[int]int(nil), false},
+	{map[int]int{}, map[int]int{}, true},
+	{map[int]int(nil), map[int]int(nil), true},
+
 	// Mismatched types
 	{1, 1.0, false},
 	{int32(1), int64(1), false},

src/pkg/reflect/deepequal.go

@@ -69,6 +69,9 @@ func deepValueEqual(v1, v2 Value, visited map[uintptr]*visit, depth int) (b bool
 		}
 		return true
 	case Slice:
+		if v1.IsNil() != v2.IsNil() {
+			return false
+		}
 		if v1.Len() != v2.Len() {
 			return false
 		}
@@ -93,6 +96,9 @@ func deepValueEqual(v1, v2 Value, visited map[uintptr]*visit, depth int) (b bool
 		}
 		return true
 	case Map:
+		if v1.IsNil() != v2.IsNil() {
+			return false
+		}
 		if v1.Len() != v2.Len() {
 			return false
 		}