[libcxx] [test] Avoid spurious test breakage in clang-cl-dll configs with newer CMake

[mstorsjo]

Summary

The pointer.volatile.pass.cpp test was already marked as XFAIL for
mingw-dll (for reasons explained in the comment above it).

The same issue also appears in clang-cl-dll when built with newer
CMake versions. (It didn't appear with older versions of CMake, as
CMake built the library with the clang-cl flag -std:c++latest when
we've requested C++ 20 - which practically built it in c++2b mode with
current clang versions. With current versions of CMake, it passes
-std:c++20 instead.)

As it succeeds/fails dependent on factors we don't
directly control, mark it as UNSUPPORTED instead of XFAIL.

This probably also needs to be backported to the 14.x branch, to
avoid spurious failures there, if the CI runners are upgraded to
a newer version of CMake.

[Mordante]

libcxx/test/std/input.output/iostream.format/output.streams/ostream.formatted/ostream.inserters.arithmetic/pointer.volatile.pass.cpp

26

Should we add a TODO that this can be removed when we require the library to build with C++2b?

[mstorsjo]

libcxx/test/std/input.output/iostream.format/output.streams/ostream.formatted/ostream.inserters.arithmetic/pointer.volatile.pass.cpp

26

Sure, that sounds like a good idea!

[mstorsjo]

Added the suggested todo.

[Mordante]

Thanks, LGTM!

libcxx/test/std/input.output/iostream.format/output.streams/ostream.formatted/ostream.inserters.arithmetic/pointer.volatile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // <ostream>
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
 
-// This test fails on MinGW DLL configurations, due to
+// This test fails in Windows DLL configurations, due to
 // __exclude_from_explicit_instantiation__ not behaving as it should in
 // combination with dllimport (https://llvm.org/PR41018), in combination
 // with running tests in c++2b mode while building the library in c++20 mode.
-// (If the library was built in c++2b mode, this test would succeed.)
-// XFAIL: target={{.+}}-windows-gnu && windows-dll
+//
+// If the library was built in c++2b mode, this test would succeed.
+//
+// Older CMake passed -std:c++latest to set C++ 20 mode on clang-cl, which
+// hid this issue. With newer CMake versions, it passes -std:c++20 which
+// makes this fail.
+//
+// Marking as UNSUPPORTED instead of XFAIL to avoid spurious failures/successes
+// depending on the version of CMake used.
+// TODO: Remove this when the library is built in c++2b mode.
+//
+// UNSUPPORTED: windows-dll
 
 // template <class charT, class traits = char_traits<charT> >
 //   class basic_ostream;
 
 // basic_ostream& operator<<(const volatile void* val);
 
 #include <ostream>
 #include <cassert>