reflect: allow unexported key in Value.MapIndex

Fixes #1748.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4444087

src/pkg/reflect/all_test.go

@@ -726,6 +726,24 @@ func TestDeepEqualComplexStructInequality(t *testing.T) {
 	}
 }
 
+type UnexpT struct {
+	m map[int]int
+}
+
+func TestDeepEqualUnexportedMap(t *testing.T) {
+	// Check that DeepEqual can look at unexported fields.
+	x1 := UnexpT{map[int]int{1: 2}}
+	x2 := UnexpT{map[int]int{1: 2}}
+	if !DeepEqual(&x1, &x2) {
+		t.Error("DeepEqual(x1, x2) = false, want true")
+	}
+
+	y1 := UnexpT{map[int]int{2: 3}}
+	if DeepEqual(&x1, &y1) {
+		t.Error("DeepEqual(x1, y1) = true, want false")
+	}
+}
+
 
 func check2ndField(x interface{}, offs uintptr, t *testing.T) {
 	s := ValueOf(x)

src/pkg/reflect/value.go

@@ -958,14 +958,19 @@ func (v Value) MapIndex(key Value) Value {
 	iv.mustBe(Map)
 	typ := iv.typ.toType()
 
+	// Do not require ikey to be exported, so that DeepEqual
+	// and other programs can use all the keys returned by
+	// MapKeys as arguments to MapIndex.  If either the map
+	// or the key is unexported, though, the result will be
+	// considered unexported.
+
 	ikey := key.internal()
-	ikey.mustBeExported()
 	ikey = convertForAssignment("reflect.Value.MapIndex", nil, typ.Key(), ikey)
 	if iv.word == 0 {
 		return Value{}
 	}
 
-	flag := iv.flag & flagRO
+	flag := (iv.flag | ikey.flag) & flagRO
 	elemType := typ.Elem()
 	elemWord, ok := mapaccess(iv.word, ikey.word)
 	if !ok {