Add support for parsing json.Number to uint64

mapstructure.go

@@ -684,16 +684,12 @@ func (d *Decoder) decodeUint(name string, data interface{}, val reflect.Value) e
 		}
 	case dataType.PkgPath() == "encoding/json" && dataType.Name() == "Number":
 		jn := data.(json.Number)
-		i, err := jn.Int64()
+		i, err := strconv.ParseUint(string(jn), 0, 64)
 		if err != nil {
 			return fmt.Errorf(
 				"error decoding json.Number into %s: %s", name, err)
 		}
-		if i < 0 && !d.config.WeaklyTypedInput {
-			return fmt.Errorf("cannot parse '%s', %d overflows uint",
-				name, i)
-		}
-		val.SetUint(uint64(i))
+		val.SetUint(i)
 	default:
 		return fmt.Errorf(
 			"'%s' expected type '%s', got unconvertible type '%s', value: '%v'",

mapstructure_test.go

@@ -24,6 +24,7 @@ type Basic struct {
 	Vdata       interface{}
 	VjsonInt    int
 	VjsonUint   uint
+	VjsonUint64 uint64
 	VjsonFloat  float64
 	VjsonNumber json.Number
 }
@@ -224,6 +225,7 @@ func TestBasicTypes(t *testing.T) {
 		"vdata":       42,
 		"vjsonInt":    json.Number("1234"),
 		"vjsonUint":   json.Number("1234"),
+		"vjsonUint64": json.Number("9223372036854775809"), // 2^63 + 1
 		"vjsonFloat":  json.Number("1234.5"),
 		"vjsonNumber": json.Number("1234.5"),
 	}
@@ -287,6 +289,10 @@ func TestBasicTypes(t *testing.T) {
 		t.Errorf("vjsonuint value should be 1234: %#v", result.VjsonUint)
 	}
 
+	if result.VjsonUint64 != 9223372036854775809 {
+		t.Errorf("vjsonuint64 value should be 9223372036854775809: %#v", result.VjsonUint64)
+	}
+
 	if result.VjsonFloat != 1234.5 {
 		t.Errorf("vjsonfloat value should be 1234.5: %#v", result.VjsonFloat)
 	}