[libc++][test] Adds format string helper.

[Mordante]

Summary

Update the formatter day tests to the new style.
Other test will be done separately.

[ldionne]

Can this be accompanied by changes in the tests proper?

[Mordante]

I created this for the formatting escaped strings, which will take some time to review. I'm also converting the test for the landed day formatter and the year formatter which will be in review soon. So from that point of view it makes sense to have it separately. If wanted I can squash this with the formatter day changes.

[Mordante]

Addresses review comments.

[Mordante]

Fix CI

[Mordante]

CI fixes.

[Mordante]

Fixes CI.

[vitaut]

One minor comment inline otherwise looks great. Much more readable than template operator()!

libcxx/test/std/time/time.syn/formatter_tests.h

33–35 ↗

(On Diff #460844)

Why is this a lambda and not a normal function? If it was the latter you could have a locale overload without inventing a new name.

[Mordante]

In D134031#3797641, @vitaut wrote:

One minor comment inline otherwise looks great. Much more readable than template operator()!

I agree. I will adjust the other tests in a separate commit.

libcxx/test/std/time/time.syn/formatter_tests.h

33–35 ↗

(On Diff #460844)

Good question. Basically since I used that pattern before. I've changed it to a normal function.

[Mordante]

Addresses review comments.

[ldionne]

libcxx/test/support/test_format_string.h

11–12

44–45

This will also avoid the comment becoming stale when/if we move to github PRs.

49

libcxx/test/support/test_format_string.h

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SUPPORT_TEST_FORMAT_STRING_HPP
+#define SUPPORT_TEST_FORMAT_STRING_HPP
+
+#include <format>
+#include <type_traits>
+
+#include "test_macros.h"
+
+#if TEST_STD_VER < 20
+#  error "The format header requires at least C++20"
+#endif
+
+// Wrapper for basic_format_string.
+//
+// This layer of indirection is used since it's not possible to use
+// std::basic_format_string<CharT, Args...> in the test function directly.
+//
+// In C++20 the basic-format-string was an exposition only type. In C++23 is
+// has been replaced with basic_format_string. Both libc++ and MSVC STL offer
+// it as an extension in C++20.
+#if TEST_STD_VER > 20 || defined(_LIBCPP_VERSION) || defined(_MSVC_STL_VERSION)
+template <class CharT, class... Args>
+using test_format_string =
+    std::conditional_t<std::same_as<CharT, char>, std::format_string<Args...>, std::wformat_string<Args...>>;
+
+#else // TEST_STD_VER > 20 || defined(_LIBCPP_VERSION) || defined( _MSVC_STL_VERSION)
+
+#  error
+"Please create a vendor specific version of the test typedef and file a review at https://reviews.llvm.org/"
+
+#endif // TEST_STD_VER > 20 || defined(_LIBCPP_VERSION) || defined( _MSVC_STL_VERSION)
+
+#endif // SUPPORT_TEST_FORMAT_STRING_HPP