Test `IsString<String>` and `IsString<StringSumHelper>`

extras/tests/Helpers/api/String.h

@@ -62,7 +62,7 @@ class String {
   size_t _maxCapacity;
 };
 
-class StringSumHelper;
+class StringSumHelper : public ::String {};
 
 inline bool operator==(const std::string& lhs, const ::String& rhs) {
   return lhs == rhs.c_str();

extras/tests/Misc/StringAdapters.cpp

@@ -87,29 +87,14 @@ TEST_CASE("custom_string") {
 }
 
 TEST_CASE("IsString<T>") {
-  SECTION("std::string") {
-    CHECK(IsString<std::string>::value == true);
-  }
-
-  SECTION("basic_string<wchar_t>") {
-    CHECK(IsString<std::basic_string<wchar_t> >::value == false);
-  }
-
-  SECTION("custom_string") {
-    CHECK(IsString<custom_string>::value == true);
-  }
-
-  SECTION("const __FlashStringHelper*") {
-    CHECK(IsString<const __FlashStringHelper*>::value == true);
-  }
-
-  SECTION("const char*") {
-    CHECK(IsString<const char*>::value == true);
-  }
-
-  SECTION("const char[]") {
-    CHECK(IsString<const char[8]>::value == true);
-  }
+  CHECK(IsString<std::string>::value == true);
+  CHECK(IsString<std::basic_string<wchar_t> >::value == false);
+  CHECK(IsString<custom_string>::value == true);
+  CHECK(IsString<const __FlashStringHelper*>::value == true);
+  CHECK(IsString<const char*>::value == true);
+  CHECK(IsString<const char[8]>::value == true);
+  CHECK(IsString< ::String>::value == true);
+  CHECK(IsString< ::StringSumHelper>::value == true);
 }
 
 TEST_CASE("stringCompare") {