Fixed JsonVariant::is<String>() (closes #763)

CHANGELOG.md

@@ -6,6 +6,7 @@ HEAD
 
 * Improved float serialization when `-fsingle-precision-constant` is used
 * Fixed `JsonVariant::is<int>()` that returned true for empty strings
+* Fixed `JsonVariant::is<String>()` (closes #763)
 
 v5.13.2
 -------

src/ArduinoJson/JsonVariant.hpp

@@ -274,9 +274,11 @@ class JsonVariant : public Internals::JsonVariantBase<JsonVariant> {
   //
   // bool is<const char*>() const;
   // bool is<char*>() const;
+  // bool is<std::string>() const;
   template <typename T>
   typename Internals::EnableIf<Internals::IsSame<T, const char *>::value ||
-                                   Internals::IsSame<T, char *>::value,
+                                   Internals::IsSame<T, char *>::value ||
+                                   Internals::StringTraits<T>::has_append,
                                bool>::type
   is() const {
     return variantIsString();
@@ -352,4 +354,4 @@ DEPRECATED("Decimal places are ignored, use the double value instead")
 inline JsonVariant double_with_n_digits(double value, uint8_t) {
   return JsonVariant(value);
 }
-}
+}  // namespace ArduinoJson

test/JsonVariant/is.cpp

@@ -58,6 +58,7 @@ void checkIsInteger(JsonVariant var) {
 
 void checkIsString(JsonVariant var) {
   REQUIRE(var.is<const char*>());
+  REQUIRE(var.is<std::string>());
 
   REQUIRE_FALSE(var.is<bool>());
   REQUIRE_FALSE(var.is<int>());