[libcxx][test] Portably test that `{w,}format_context` is a specialization of `basic_format_context`

[• Quuxplusone]

Seems uncontroversial to me.

libcxx/test/std/utilities/format/format.formatter/format.context/types.compile.pass.cpp

110–122

Primarily just to rename the lowercase template parameter charT.

[CaseyCarter]

Address review

[CaseyCarter]

libcxx/test/std/utilities/format/format.formatter/format.context/types.compile.pass.cpp

110–122

I when with CharT to be consistent with the rest of the file (which I should have done in the first place, instead of using charT to be consistent with the working draft).

[Mordante]

Thanks for the fix. LGTM!

libcxx/test/std/utilities/format/format.formatter/format.context/types.compile.pass.cpp

110–122

I too prefer CharT since that's consistent with the other <format> tests.

libcxx/test/std/utilities/format/format.formatter/format.context/types.compile.pass.cpp

 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   test<std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>();
 #endif
   test<std::back_insert_iterator<std::basic_string<char8_t>>, char8_t>();
   test<std::back_insert_iterator<std::basic_string<char16_t>>, char16_t>();
   test<std::back_insert_iterator<std::basic_string<char32_t>>, char32_t>();
 }
 
-static_assert(std::is_same_v<
-              std::format_context,
-              std::basic_format_context<
-                  std::back_insert_iterator<std::basic_string<char>>, char>>);
+template <class, class>
+constexpr bool is_basic_format_context_specialization = false;
+template <class It, class CharT>
+constexpr bool is_basic_format_context_specialization<std::basic_format_context<It, CharT>, CharT> = true;
+
+static_assert(is_basic_format_context_specialization<std::format_context, char>);
+LIBCPP_STATIC_ASSERT(
+    std::is_same_v<
+
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
-static_assert(
+static_assert(is_basic_format_context_specialization<std::wformat_context, wchar_t>);
+LIBCPP_STATIC_ASSERT(
     std::is_same_v<
         std::wformat_context,
         std::basic_format_context<
             std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>>);
 #endif
 
 // Required for MSVC internal test runner compatibility.
 int main(int, char**) { return 0; }