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[libc++] Fix C++23 standard modules when using with clang-cl on Windows #148992

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[libc++] Fix C++23 standard modules when using with clang-cl on Windows #148992

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ed2c4e4
fix libc++ std modules compilation on Windows using clang-cl
siradam7th Jul 15, 2025
74d49fc
fix python file encoding error on Windows
siradam7th Jul 16, 2025
bec4906
move define '_BUILD_STD_MODULE' to 'generate_libcxx_cppm_in.py'
siradam7th Jul 16, 2025
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5 changes: 5 additions & 0 deletions libcxx/include/__new/new_handler.h
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Expand Up @@ -17,6 +17,11 @@

#if defined(_LIBCPP_ABI_VCRUNTIME)
# include <new.h>
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How does the MSVC STL get their own definition for get_new_handler()? It seems really brittle to include <new.h> but have our definition for get_new_handler(). We should either declare all of them (set_new_handler(), the new_handler alias, etc) or rely on <new.h> to provide them, but not a mixture of both.

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MSVC STL just (additionally) declares it in <new>. The declaration is not available from <new.h> or <vcruntime_new.h>.

// <new.h> does not define 'get_new_handler'
// which makes the 'std' module build fail, this fixes it
namespace std {
_LIBCPP_EXPORTED_FROM_ABI new_handler get_new_handler() _NOEXCEPT;
}
#else
_LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
typedef void (*new_handler)();
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6 changes: 4 additions & 2 deletions libcxx/modules/std.compat/ctime.inc
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Expand Up @@ -14,15 +14,17 @@ export {

using ::timespec _LIBCPP_USING_IF_EXISTS;
using ::tm _LIBCPP_USING_IF_EXISTS;

using ::asctime _LIBCPP_USING_IF_EXISTS;
using ::clock _LIBCPP_USING_IF_EXISTS;
using ::strftime _LIBCPP_USING_IF_EXISTS;

#ifndef _LIBCPP_ABI_VCRUNTIME
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Are you simply leaving these functions missing when using UCRT?

Also, it seem that we should modify this line libcxx/utils/libcxx/test/features.py to enable the tests:

llvm-project/libcxx/utils/libcxx/test/features.py

Line 330 in 968d38d

or ("_WIN32" in compilerMacros(cfg) and not _mingwSupportsModules(cfg))

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I'm looking into it, however it seems that moving the definition '_BUILD_STD_MODULE' triggered some other weird errors, I'll do more testing tomorrow and see...

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Aha, these lines should be removed. Now get_new_handler.pass.cpp should always pass.

llvm-project/libcxx/test/std/language.support/support.dynamic/alloc.errors/set.new.handler/get_new_handler.pass.cpp

Lines 11 to 17 in 653872f

// FIXME: When libc++ is linked against vcruntime (i.e. the default config in
// MSVC mode), the declarations of std::set_new_handler and std::get_new_handler
// are provided by vcruntime/UCRT's new.h. However, that header actually only
// declares set_new_handler - it's missing a declaration of get_new_handler.
// XFAIL: msvc && stdlib=libc++

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It looks like just defining the macro is not enough, I have resorted to defining the actual functions as inline, I'll push the changes once I made sure that everything works with both std and std.compact.

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The _LIBCPP_USING_IF_EXISTS attribute is intended to make things work when the declarations are not available, and avoid precisely these #ifdef blocks. What's the issue here?

using ::ctime _LIBCPP_USING_IF_EXISTS;
using ::difftime _LIBCPP_USING_IF_EXISTS;
using ::gmtime _LIBCPP_USING_IF_EXISTS;
using ::localtime _LIBCPP_USING_IF_EXISTS;
using ::mktime _LIBCPP_USING_IF_EXISTS;
using ::strftime _LIBCPP_USING_IF_EXISTS;
using ::time _LIBCPP_USING_IF_EXISTS;
using ::timespec_get _LIBCPP_USING_IF_EXISTS;
#endif
} // export
4 changes: 4 additions & 0 deletions libcxx/modules/std.cppm.in
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Expand Up @@ -7,6 +7,10 @@
//
//===----------------------------------------------------------------------===//

#ifdef _LIBCPP_ABI_VCRUNTIME
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I assume this change is temporary?

#define _BUILD_STD_MODULE
#endif

// WARNING, this entire header is generated by
// utils/generate_libcxx_cppm_in.py
// DO NOT MODIFY!
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6 changes: 4 additions & 2 deletions libcxx/modules/std/ctime.inc
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Expand Up @@ -14,15 +14,17 @@ export namespace std {

using std::timespec _LIBCPP_USING_IF_EXISTS;
using std::tm _LIBCPP_USING_IF_EXISTS;

using std::asctime _LIBCPP_USING_IF_EXISTS;
using std::clock _LIBCPP_USING_IF_EXISTS;
using std::strftime _LIBCPP_USING_IF_EXISTS;

#ifndef _LIBCPP_ABI_VCRUNTIME
using std::ctime _LIBCPP_USING_IF_EXISTS;
using std::difftime _LIBCPP_USING_IF_EXISTS;
using std::gmtime _LIBCPP_USING_IF_EXISTS;
using std::localtime _LIBCPP_USING_IF_EXISTS;
using std::mktime _LIBCPP_USING_IF_EXISTS;
using std::strftime _LIBCPP_USING_IF_EXISTS;
using std::time _LIBCPP_USING_IF_EXISTS;
using std::timespec_get _LIBCPP_USING_IF_EXISTS;
#endif
} // namespace std
6 changes: 5 additions & 1 deletion libcxx/utils/generate_libcxx_cppm_in.py
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Expand Up @@ -13,7 +13,7 @@


def write_file(module):
with open(libcxx_root / "modules" / f"{module}.cppm.in", "w") as module_cpp_in:
with open(libcxx_root / "modules" / f"{module}.cppm.in", "w", encoding='utf-8') as module_cpp_in:
module_cpp_in.write(
"""\
// -*- C++ -*-
Expand All @@ -25,6 +25,10 @@ def write_file(module):
//
//===----------------------------------------------------------------------===//

#ifdef _LIBCPP_ABI_VCRUNTIME
#define _BUILD_STD_MODULE
#endif

// WARNING, this entire header is generated by
// utils/generate_libcxx_cppm_in.py
// DO NOT MODIFY!
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