encoding/json: encode map key is of string kind, decode only of string type

Allows encoding and decoding of maps with key of string kind, not just string type.
Fixes #3519.

R=rsc, dave
CC=golang-dev
https://golang.org/cl/6943047

src/pkg/encoding/json/decode.go

@@ -430,9 +430,9 @@ func (d *decodeState) object(v reflect.Value) {
 	// Check type of target: struct or map[string]T
 	switch v.Kind() {
 	case reflect.Map:
-		// map must have string type
+		// map must have string kind
 		t := v.Type()
-		if t.Key() != reflect.TypeOf("") {
+		if t.Key().Kind() != reflect.String {
 			d.saveError(&UnmarshalTypeError{"object", v.Type()})
 			break
 		}
@@ -536,10 +536,12 @@ func (d *decodeState) object(v reflect.Value) {
 		} else {
 			d.value(subv)
 		}
+
 		// Write value back to map;
 		// if using struct, subv points into struct already.
 		if v.Kind() == reflect.Map {
-			v.SetMapIndex(reflect.ValueOf(key), subv)
+			kv := reflect.ValueOf(key).Convert(v.Type().Key())
+			v.SetMapIndex(kv, subv)
 		}
 
 		// Next token must be , or }.

src/pkg/encoding/json/decode_test.go

@@ -1000,3 +1000,28 @@ func TestUnmarshalNulls(t *testing.T) {
 		t.Errorf("Unmarshal of null values affected primitives")
 	}
 }
+
+func TestStringKind(t *testing.T) {
+	type stringKind string
+	type aMap map[stringKind]int
+
+	var m1, m2 map[stringKind]int
+	m1 = map[stringKind]int{
+		"foo": 42,
+	}
+
+	data, err := Marshal(m1)
+	if err != nil {
+		t.Errorf("Unexpected error marshalling: %v", err)
+	}
+
+	err = Unmarshal(data, &m2)
+	if err != nil {
+		t.Errorf("Unexpected error unmarshalling: %v", err)
+	}
+
+	if !reflect.DeepEqual(m1, m2) {
+		t.Error("Items should be equal after encoding and then decoding")
+	}
+
+}