encoding/json: support maps with integer keys

This change makes encoding and decoding support integer types in map
keys, converting to/from JSON string keys.

JSON object keys are still sorted lexically, even though the keys may be
integer strings.

For backwards-compatibility, the existing Text(Un)Marshaler support for
map keys (added in CL 20356) does not take precedence over the default
encoding for string types. There is no such concern for integer types,
so integer map key encoding is only used as a fallback if the map key
type is not a Text(Un)Marshaler.

Fixes #12529.

Change-Id: I7e68c34f9cd19704b1d233a9862da15fabf0908a
Reviewed-on: https://go-review.googlesource.com/22060
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/json/decode.go

@@ -62,10 +62,10 @@ import (
 // the additional Go array elements are set to zero values.
 //
 // To unmarshal a JSON object into a map, Unmarshal first establishes a map to
-// use, If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal
+// use. If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal
 // reuses the existing map, keeping existing entries. Unmarshal then stores key-
-// value pairs from the JSON object into the map.  The map's key type must
-// either be a string or implement encoding.TextUnmarshaler.
+// value pairs from the JSON object into the map. The map's key type must
+// either be a string, an integer, or implement encoding.TextUnmarshaler.
 //
 // If a JSON value is not appropriate for a given target type,
 // or if a JSON number overflows the target type, Unmarshal
@@ -581,17 +581,24 @@ func (d *decodeState) object(v reflect.Value) {
 
 	// Check type of target:
 	//   struct or
-	//   map[string]T or map[encoding.TextUnmarshaler]T
+	//   map[T1]T2 where T1 is string, an integer type,
+	//             or an encoding.TextUnmarshaler
 	switch v.Kind() {
 	case reflect.Map:
-		// Map key must either have string kind or be an encoding.TextUnmarshaler.
+		// Map key must either have string kind, have an integer kind,
+		// or be an encoding.TextUnmarshaler.
 		t := v.Type()
-		if t.Key().Kind() != reflect.String &&
-			!reflect.PtrTo(t.Key()).Implements(textUnmarshalerType) {
-			d.saveError(&UnmarshalTypeError{"object", v.Type(), int64(d.off)})
-			d.off--
-			d.next() // skip over { } in input
-			return
+		switch t.Key().Kind() {
+		case reflect.String,
+			reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
+			reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		default:
+			if !reflect.PtrTo(t.Key()).Implements(textUnmarshalerType) {
+				d.saveError(&UnmarshalTypeError{"object", v.Type(), int64(d.off)})
+				d.off--
+				d.next() // skip over { } in input
+				return
+			}
 		}
 		if v.IsNil() {
 			v.Set(reflect.MakeMap(t))
@@ -696,13 +703,32 @@ func (d *decodeState) object(v reflect.Value) {
 			var kv reflect.Value
 			switch {
 			case kt.Kind() == reflect.String:
-				kv = reflect.ValueOf(key).Convert(v.Type().Key())
+				kv = reflect.ValueOf(key).Convert(kt)
 			case reflect.PtrTo(kt).Implements(textUnmarshalerType):
 				kv = reflect.New(v.Type().Key())
 				d.literalStore(item, kv, true)
 				kv = kv.Elem()
 			default:
-				panic("json: Unexpected key type") // should never occur
+				switch kt.Kind() {
+				case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+					s := string(key)
+					n, err := strconv.ParseInt(s, 10, 64)
+					if err != nil || reflect.Zero(kt).OverflowInt(n) {
+						d.saveError(&UnmarshalTypeError{"number " + s, kt, int64(start + 1)})
+						return
+					}
+					kv = reflect.ValueOf(n).Convert(kt)
+				case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+					s := string(key)
+					n, err := strconv.ParseUint(s, 10, 64)
+					if err != nil || reflect.Zero(kt).OverflowUint(n) {
+						d.saveError(&UnmarshalTypeError{"number " + s, kt, int64(start + 1)})
+						return
+					}
+					kv = reflect.ValueOf(n).Convert(kt)
+				default:
+					panic("json: Unexpected key type") // should never occur
+				}
 			}
 			v.SetMapIndex(kv, subv)
 		}