[libc++][test] Improves C++ Standard filtering. (#89499)

Adds a new lit directive to improve C++ Standard filtering. This is
based on the

[Discourse](https://discourse.llvm.org/t/rfc-improving-c-standard-filtering-in-the-lit-tests/78474)
discussion.

Author: mordante

libcxx/docs/TestingLibcxx.rst

@@ -435,6 +435,38 @@ writing tests easier. See `libc++-specific Lit Directives`_ for more information
        extension.)
 
 
+C++ Standard version tests
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Historically libc++ tests used to filter the tests for C++ Standard versions
+with lit directives like:
+
+.. code-block:: cpp
+
+   // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+With C++ Standards released every 3 years, this solution is not scalable.
+Instead use:
+
+.. code-block:: cpp
+
+   // UNSUPPORTED: std-at-least-c++26
+
+There is no corresponding ``std-at-most-c++23``. This could be useful when
+tests are only valid for a small set of standard versions. For example, a
+deprecation test is only valid when the feature is deprecated until it is
+removed from the Standard. These tests should be written like:
+
+.. code-block:: cpp
+
+   // REQUIRES: c++17 || c++20 || c++23
+
+.. note::
+
+   There are a lot of tests with the first style, these can remain as they are.
+   The new style is only intended to be used for new tests.
+
+
 Benchmarks
 ==========
 

libcxx/test/std/input.output/iostream.format/print.fun/includes.compile.pass.cpp

@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// REQUIRES: std-at-least-c++23
 // UNSUPPORTED: no-filesystem
 // UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
 

libcxx/test/std/input.output/iostream.format/print.fun/no_file_description.pass.cpp

@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// REQUIRES: std-at-least-c++23
 // UNSUPPORTED: no-filesystem
 // UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
 

libcxx/test/std/localization/locale.stdcvt/depr.verify.cpp

@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14, c++26
+// REQUIRES: c++17 || c++20 || c++23
 // UNSUPPORTED: no-wide-characters
 
 // <codecvt>

libcxx/test/std/localization/locales/locale.convenience/conversions/conversions.buffer/depr.verify.cpp

@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14, c++26
+// REQUIRES: c++17 || c++20 || c++23
 
 // XFAIL: no-wide-characters
 

libcxx/test/std/localization/locales/locale.convenience/conversions/conversions.string/depr.verify.cpp

@@ -8,7 +8,7 @@
 
 // ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX26_REMOVED_WSTRING_CONVERT
 
-// UNSUPPORTED: c++03, c++11, c++14, c++26
+// REQUIRES: c++17 || c++20 || c++23
 // UNSUPPORTED: no-wide-characters
 
 // <codecvt>

libcxx/test/std/strings/basic.string/string.capacity/reserve.deprecated_in_cxx20.verify.cpp

@@ -10,7 +10,7 @@
 
 // void reserve(); // Deprecated in C++20
 
-// UNSUPPORTED: c++03, c++11, c++14, c++17, c++26
+// REQUIRES: c++20 || c++23
 
 #include <string>
 

libcxx/utils/libcxx/test/params.py

@@ -186,7 +186,8 @@ def getSuitableClangTidy(cfg):
             AddFeature(std),
             AddSubstitution("%{cxx_std}", re.sub(r"\+", "x", std)),
             AddCompileFlag(lambda cfg: getStdFlag(cfg, std)),
-        ],
+        ]
+        + [AddFeature(f"std-at-least-{s}") for s in _allStandards if s <= std],
     ),
     Parameter(
         name="optimization",