Add extern "C++" as a temporary workaround for #include/import …

…coexistence (#4154)

.clang-format

@@ -265,6 +265,8 @@ SpaceBeforeParensOptions:
 # NOTE: _STD_BEGIN, _STD_END, etc. aren't macros for complete statements, but telling
 # clang-format that they are produces the behavior that we want (with no block indentation).
 StatementMacros:
+  - _EXTERN_CXX_WORKAROUND
+  - _END_EXTERN_CXX_WORKAROUND
   - _STD_BEGIN
   - _STD_END
   - _STDEXT_BEGIN

stl/inc/cmath

@@ -33,6 +33,7 @@ _STL_DISABLE_CLANG_WARNINGS
 #pragma push_macro("new")
 #undef new
 
+_EXTERN_CXX_WORKAROUND
 _NODISCARD _Check_return_ inline float acos(_In_ float _Xx) noexcept /* strengthened */ {
     return _CSTD acosf(_Xx);
 }
@@ -552,6 +553,7 @@ _NODISCARD _Check_return_ inline long double trunc(_In_ long double _Xx) noexcep
     return _CSTD truncl(_Xx);
 #endif // ^^^ intrinsics unavailable ^^^
 }
+_END_EXTERN_CXX_WORKAROUND
 
 _STD_BEGIN
 template <class _Ty1, class _Ty2>
@@ -560,6 +562,7 @@ using _Common_float_type_t = conditional_t<is_same_v<_Ty1, long double> || is_sa
         double>>; // find type for two-argument math function
 _STD_END
 
+_EXTERN_CXX_WORKAROUND
 template <class _Ty, _STD enable_if_t<_STD is_integral_v<_Ty>, int> = 0>
 double frexp(_Ty _Value, _Out_ int* const _Exp) noexcept /* strengthened */ {
     return _CSTD frexp(static_cast<double>(_Value), _Exp);
@@ -709,6 +712,7 @@ _GENERIC_MATH2(fmin)
 #undef _GENERIC_MATH2
 #undef _GENERIC_MATH2I
 #undef _HAS_CMATH_INTRINSICS
+_END_EXTERN_CXX_WORKAROUND
 
 _STD_BEGIN
 _EXPORT_STD using _CSTD abs;

stl/inc/cstddef

@@ -98,7 +98,9 @@ _NODISCARD _MSVC_INTRINSIC constexpr _IntType to_integer(const byte _Arg) noexce
 
 _STD_END
 
+_EXTERN_CXX_WORKAROUND
 using _STD max_align_t; // intentional, for historical reasons
+_END_EXTERN_CXX_WORKAROUND
 
 // TRANSITION, non-_Ugly attribute tokens
 #pragma pop_macro("intrinsic")

stl/inc/cstdlib

@@ -18,6 +18,7 @@ _STL_DISABLE_CLANG_WARNINGS
 #pragma push_macro("new")
 #undef new
 
+_EXTERN_CXX_WORKAROUND
 // <stdlib.h> has abs(long) and abs(long long)
 _NODISCARD _Check_return_ inline double abs(_In_ double _Xx) noexcept /* strengthened */ {
     return _CSTD fabs(_Xx);
@@ -30,6 +31,7 @@ _NODISCARD _Check_return_ inline float abs(_In_ float _Xx) noexcept /* strengthe
 _NODISCARD _Check_return_ inline long double abs(_In_ long double _Xx) noexcept /* strengthened */ {
     return _CSTD fabsl(_Xx);
 }
+_END_EXTERN_CXX_WORKAROUND
 
 _STD_BEGIN
 _EXPORT_STD using _CSTD size_t;

stl/inc/ctime

@@ -29,44 +29,44 @@ _EXPORT_STD using _CSTD strftime;
 _EXPORT_STD using _CSTD timespec;
 #endif // _HAS_CXX17
 
-#ifdef _BUILD_STD_MODULE // TRANSITION, OS-33790456
+#ifdef _BUILD_STD_MODULE // TRANSITION, OS-33790456; `template <int = 0>` avoids ambiguity
 _STL_DISABLE_DEPRECATED_WARNING
 
-_EXPORT_STD
+_EXPORT_STD template <int = 0>
 _Check_return_ _CRT_INSECURE_DEPRECATE(ctime_s) inline char* __CRTDECL ctime(_In_ const time_t* const _Time) noexcept
 /* strengthened */ {
     return _CSTD _ctime64(_Time);
 }
 
-_EXPORT_STD
+_EXPORT_STD template <int = 0>
 _Check_return_ inline double __CRTDECL difftime(_In_ const time_t _Time1, _In_ const time_t _Time2) noexcept
 /* strengthened */ {
     return _CSTD _difftime64(_Time1, _Time2);
 }
 
-_EXPORT_STD
+_EXPORT_STD template <int = 0>
 _Check_return_ _CRT_INSECURE_DEPRECATE(gmtime_s) inline tm* __CRTDECL gmtime(_In_ const time_t* const _Time) noexcept
 /* strengthened */ {
     return _CSTD _gmtime64(_Time);
 }
 
-_EXPORT_STD
+_EXPORT_STD template <int = 0>
 _CRT_INSECURE_DEPRECATE(localtime_s)
 inline tm* __CRTDECL localtime(_In_ const time_t* const _Time) noexcept /* strengthened */ {
     return _CSTD _localtime64(_Time);
 }
 
-_EXPORT_STD
+_EXPORT_STD template <int = 0>
 _Check_return_opt_ inline time_t __CRTDECL mktime(_Inout_ tm* const _Tm) noexcept /* strengthened */ {
     return _CSTD _mktime64(_Tm);
 }
 
-_EXPORT_STD
+_EXPORT_STD template <int = 0>
 inline time_t __CRTDECL time(_Out_opt_ time_t* const _Time) noexcept /* strengthened */ {
     return _CSTD _time64(_Time);
 }
 
-_EXPORT_STD
+_EXPORT_STD template <int = 0>
 _Check_return_ inline int __CRTDECL timespec_get(_Out_ timespec* const _Ts, _In_ const int _Base) noexcept
 /* strengthened */ {
     return _CSTD _timespec64_get(reinterpret_cast<_timespec64*>(_Ts), _Base);

stl/inc/cwchar

@@ -18,7 +18,9 @@ _STL_DISABLE_CLANG_WARNINGS
 #pragma push_macro("new")
 #undef new
 
+_EXTERN_CXX_WORKAROUND
 using _Mbstatet = mbstate_t;
+_END_EXTERN_CXX_WORKAROUND
 
 _STD_BEGIN
 #pragma warning(push)

stl/inc/xfilesystem_abi.h

@@ -78,7 +78,9 @@ enum class __std_fs_file_attr : unsigned long {
 };
 } // extern "C"
 
+_EXTERN_CXX_WORKAROUND
 _BITMASK_OPS(_EMPTY_ARGUMENT, __std_fs_file_attr)
+_END_EXTERN_CXX_WORKAROUND
 
 extern "C" {
 enum class __std_fs_reparse_tag : unsigned long {
@@ -123,7 +125,9 @@ enum class __std_fs_stats_flags : unsigned long {
 };
 } // extern "C"
 
+_EXTERN_CXX_WORKAROUND
 _BITMASK_OPS(_EMPTY_ARGUMENT, __std_fs_stats_flags)
+_END_EXTERN_CXX_WORKAROUND
 
 extern "C" {
 struct __std_fs_stats {
@@ -199,7 +203,9 @@ enum class __std_access_rights : unsigned long {
 };
 } // extern "C"
 
+_EXTERN_CXX_WORKAROUND
 _BITMASK_OPS(_EMPTY_ARGUMENT, __std_access_rights)
+_END_EXTERN_CXX_WORKAROUND
 
 extern "C" {
 enum class __std_fs_file_flags : unsigned long {
@@ -209,7 +215,9 @@ enum class __std_fs_file_flags : unsigned long {
 };
 } // extern "C"
 
+_EXTERN_CXX_WORKAROUND
 _BITMASK_OPS(_EMPTY_ARGUMENT, __std_fs_file_flags)
+_END_EXTERN_CXX_WORKAROUND
 
 extern "C" {
 enum class __std_fs_file_handle : intptr_t { _Invalid = -1 };
@@ -236,7 +244,9 @@ enum class __std_fs_copy_options {
 };
 } // extern "C"
 
+_EXTERN_CXX_WORKAROUND
 _BITMASK_OPS(_EMPTY_ARGUMENT, __std_fs_copy_options)
+_END_EXTERN_CXX_WORKAROUND
 
 extern "C" {
 _NODISCARD __std_ulong_and_error __stdcall __std_fs_get_full_path_name(_In_z_ const wchar_t* _Source,

stl/inc/yvals_core.h

@@ -1941,16 +1941,49 @@ compiler option, or define _ALLOW_RTCc_IN_STL to suppress this error.
 #define _STRINGIZE(x)   _STRINGIZEX(x)
 #define _EMPTY_ARGUMENT // for empty macro argument
 
-#define _STD_BEGIN namespace std {
-#define _STD_END   }
-#define _STD       ::std::
-#define _CHRONO    ::std::chrono::
-#define _RANGES    ::std::ranges::
+// extern "C++" attaches declarations to the global module, see N4964 [module.unit]/7.2.
+// It has no effect in C++14/17.
+
+// In the STL's headers (which might be used to build the named module std), we unconditionally
+// and directly mark declarations of our separately compiled machinery as extern "C++", allowing
+// the named module to work with the separately compiled code (which is always built classically).
+
+// TRANSITION: _USE_EXTERN_CXX_EVERYWHERE_FOR_STL controls whether we also wrap the STL's
+// header-only code in this linkage-specification, as a temporary workaround to allow
+// the named module to coexist with classic includes in the same translation unit.
+
+#ifndef _USE_EXTERN_CXX_EVERYWHERE_FOR_STL
+#define _USE_EXTERN_CXX_EVERYWHERE_FOR_STL _HAS_CXX20
+#endif // ^^^ !defined(_USE_EXTERN_CXX_EVERYWHERE_FOR_STL) ^^^
+
+#if _USE_EXTERN_CXX_EVERYWHERE_FOR_STL
+#define _EXTERN_CXX_WORKAROUND     extern "C++" {
+#define _END_EXTERN_CXX_WORKAROUND }
+#else // ^^^ _USE_EXTERN_CXX_EVERYWHERE_FOR_STL / !_USE_EXTERN_CXX_EVERYWHERE_FOR_STL vvv
+#define _EXTERN_CXX_WORKAROUND
+#define _END_EXTERN_CXX_WORKAROUND
+#endif // ^^^ !_USE_EXTERN_CXX_EVERYWHERE_FOR_STL ^^^
+
+#define _STD_BEGIN         \
+    _EXTERN_CXX_WORKAROUND \
+    namespace std {
+#define _STD_END \
+    }            \
+    _END_EXTERN_CXX_WORKAROUND
+
+#define _STD    ::std::
+#define _CHRONO ::std::chrono::
+#define _RANGES ::std::ranges::
 
 // We use the stdext (standard extension) namespace to contain extensions that are not part of the current standard
-#define _STDEXT_BEGIN namespace stdext {
-#define _STDEXT_END   }
-#define _STDEXT       ::stdext::
+#define _STDEXT_BEGIN      \
+    _EXTERN_CXX_WORKAROUND \
+    namespace stdext {
+#define _STDEXT_END \
+    }               \
+    _END_EXTERN_CXX_WORKAROUND
+
+#define _STDEXT ::stdext::
 
 #define _CSTD ::
 

tests/std/tests/P2465R3_standard_library_modules/custom_format.py

@@ -17,10 +17,12 @@ def getBuildSteps(self, test, litConfig, shared):
         testCpp = test.getSourcePath()
         sourceDir = os.path.dirname(testCpp)
         test2Cpp = os.path.join(sourceDir, 'test2.cpp')
+        test3Cpp = os.path.join(sourceDir, 'test3.cpp')
+        test4Cpp = os.path.join(sourceDir, 'test4.cpp')
         classicCpp = os.path.join(sourceDir, 'classic.cpp')
 
         # Dependency order is important here:
-        inputPaths = [stdIxx, stdCompatIxx, testCpp, test2Cpp, classicCpp]
+        inputPaths = [stdIxx, stdCompatIxx, testCpp, test2Cpp, test3Cpp, test4Cpp, classicCpp]
 
         cmd = [test.cxx, *inputPaths, *test.flags, *test.compileFlags]
 

tests/std/tests/P2465R3_standard_library_modules/custombuild.pl

@@ -16,7 +16,7 @@ ()
     my $stdCompatIxx = "$stlModulesDir\\std.compat.ixx";
 
     # Dependency order is important here:
-    my @inputPaths = ($stdIxx, $stdCompatIxx, "test.cpp", "test2.cpp", "classic.cpp");
+    my @inputPaths = ($stdIxx, $stdCompatIxx, "test.cpp", "test2.cpp", "test3.cpp", "test4.cpp", "classic.cpp");
 
     Run::ExecuteCL(join(" ", @inputPaths, "/Fe$cwd.exe"));
 }

tests/std/tests/P2465R3_standard_library_modules/test.cpp

@@ -13,12 +13,16 @@ import std;
 void prepare_test_environment();
 void all_std_cmeow_tests();
 void test_module_std_compat();
+void test_include_all_then_import_std();
+void test_include_all_then_import_std_compat();
 
 int main() {
     prepare_test_environment(); // defined in classic.cpp
     all_cpp_header_tests(); // defined in test_header_units_and_modules.hpp
     all_std_cmeow_tests(); // defined below
     test_module_std_compat(); // defined in test2.cpp
+    test_include_all_then_import_std(); // defined in test3.cpp
+    test_include_all_then_import_std_compat(); // defined in test4.cpp
 }
 
 void test_std_cassert() {

tests/std/tests/P2465R3_standard_library_modules/test3.cpp

@@ -0,0 +1,30 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifdef _MSVC_INTERNAL_TESTING // TRANSITION, VS 2022 17.9 Preview 3
+#include <__msvc_all_public_headers.hpp>
+#else // ^^^ no workaround / workaround vvv
+#include <assert.h> // intentionally not <cassert>
+#endif // ^^^ workaround ^^^
+
+import std;
+
+// INTENTIONALLY AVOIDED: using namespace std;
+
+void test_include_all_then_import_std() {
+    // Verify that std::vector and std::ranges algorithms are available:
+    std::vector<int> v{31, 41, 59, 26, 53, 58, 97, 93};
+    assert(!std::ranges::is_sorted(v));
+    std::ranges::sort(v);
+    assert(std::ranges::is_sorted(v));
+    const std::vector<int> sorted{26, 31, 41, 53, 58, 59, 93, 97};
+    assert(v == sorted);
+
+    // Verify that the Sufficient Additional Overloads are available for std::sqrt():
+    assert(std::sqrt(25.0) == 5.0);
+    assert(std::sqrt(25.0f) == 5.0f);
+    assert(std::sqrt(25) == 5.0);
+    static_assert(std::is_same_v<decltype(std::sqrt(25.0)), double>);
+    static_assert(std::is_same_v<decltype(std::sqrt(25.0f)), float>);
+    static_assert(std::is_same_v<decltype(std::sqrt(25)), double>);
+}

tests/std/tests/P2465R3_standard_library_modules/test4.cpp

@@ -0,0 +1,38 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifdef _MSVC_INTERNAL_TESTING // TRANSITION, VS 2022 17.9 Preview 3
+#include <__msvc_all_public_headers.hpp>
+#else // ^^^ no workaround / workaround vvv
+#include <assert.h> // intentionally not <cassert>
+#endif // ^^^ workaround ^^^
+
+import std.compat;
+
+// INTENTIONALLY AVOIDED: using namespace std;
+
+void test_include_all_then_import_std_compat() {
+    // Verify that std::vector and std::ranges algorithms are available:
+    std::vector<int> v{31, 41, 59, 26, 53, 58, 97, 93};
+    assert(!std::ranges::is_sorted(v));
+    std::ranges::sort(v);
+    assert(std::ranges::is_sorted(v));
+    const std::vector<int> sorted{26, 31, 41, 53, 58, 59, 93, 97};
+    assert(v == sorted);
+
+    // Verify that the Sufficient Additional Overloads are available for std::sqrt():
+    assert(std::sqrt(25.0) == 5.0);
+    assert(std::sqrt(25.0f) == 5.0f);
+    assert(std::sqrt(25) == 5.0);
+    static_assert(std::is_same_v<decltype(std::sqrt(25.0)), double>);
+    static_assert(std::is_same_v<decltype(std::sqrt(25.0f)), float>);
+    static_assert(std::is_same_v<decltype(std::sqrt(25)), double>);
+
+    // Verify that the Sufficient Additional Overloads are available for ::sqrt():
+    assert(::sqrt(25.0) == 5.0);
+    assert(::sqrt(25.0f) == 5.0f);
+    assert(::sqrt(25) == 5.0);
+    static_assert(std::is_same_v<decltype(::sqrt(25.0)), double>);
+    static_assert(std::is_same_v<decltype(::sqrt(25.0f)), float>);
+    static_assert(std::is_same_v<decltype(::sqrt(25)), double>);
+}