C++20 Standard Library Modules! (#3977)

After discussion and buy-in from other Standard Library maintainers, we've decided to support the named modules (`std` and `std.compat`) from C++23 in C++20 mode.

Author: StephanTLavavej

stl/inc/ctime

@@ -29,7 +29,7 @@ _EXPORT_STD using _CSTD strftime;
 _EXPORT_STD using _CSTD timespec;
 #endif // _HAS_CXX17
 
-#if _HAS_CXX23 && defined(_BUILD_STD_MODULE) // TRANSITION, OS-33790456
+#ifdef _BUILD_STD_MODULE // TRANSITION, OS-33790456
 _STL_DISABLE_DEPRECATED_WARNING
 
 _EXPORT_STD

stl/inc/yvals_core.h

@@ -292,6 +292,7 @@
 // P2418R2 Add Support For std::generator-like Types To std::format
 // P2419R2 Clarify Handling Of Encodings In Localized Formatting Of chrono Types
 // P2432R1 Fix istream_view
+// P2465R3 Standard Library Modules std And std.compat
 // P2508R1 basic_format_string, format_string, wformat_string
 // P2520R0 move_iterator<T*> Should Be A Random-Access Iterator
 // P2538R1 ADL-Proof projected
@@ -365,7 +366,6 @@
 // P2443R1 views::chunk_by
 // P2445R1 forward_like()
 // P2446R2 views::as_rvalue
-// P2465R3 Standard Library Modules std And std.compat
 // P2467R1 ios_base::noreplace: Exclusive Mode For fstreams
 // P2474R2 views::repeat
 // P2494R2 Relaxing Range Adaptors To Allow Move-Only Types
@@ -928,11 +928,14 @@ _EMIT_STL_ERROR(STL1001, "Unexpected compiler version, expected MSVC 19.36 or ne
 #endif // ^^^ inline (not constexpr) in C++20 and earlier ^^^
 
 // P2465R3 Standard Library Modules std And std.compat
-#if _HAS_CXX23 && defined(_BUILD_STD_MODULE)
+#ifdef _BUILD_STD_MODULE
+#if !_HAS_CXX20
+#error The Standard Library Modules are available only with C++20 or later.
+#endif // ^^^ !_HAS_CXX20 ^^^
 #define _EXPORT_STD export
-#else // _HAS_CXX23 && defined(_BUILD_STD_MODULE)
+#else // ^^^ defined(_BUILD_STD_MODULE) / !defined(_BUILD_STD_MODULE) vvv
 #define _EXPORT_STD
-#endif // _HAS_CXX23 && defined(_BUILD_STD_MODULE)
+#endif // ^^^ !defined(_BUILD_STD_MODULE) ^^^
 
 // P0607R0 Inline Variables For The STL
 #if _HAS_CXX17
@@ -1752,6 +1755,10 @@ _EMIT_STL_ERROR(STL1004, "C++98 unexpected() is incompatible with C++23 unexpect
 #define __cpp_lib_list_remove_return_type 201806L
 #define __cpp_lib_math_constants          201907L
 
+#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, Clang and EDG support for modules
+#define __cpp_lib_modules 202207L
+#endif // !defined(__clang__) && !defined(__EDG__)
+
 #ifdef __cpp_lib_concepts
 #define __cpp_lib_move_iterator_concept 202207L
 #endif // defined(__cpp_lib_concepts)
@@ -1793,15 +1800,10 @@ _EMIT_STL_ERROR(STL1004, "C++98 unexpected() is incompatible with C++23 unexpect
 #define __cpp_lib_formatters        202302L
 #endif // defined(__cpp_lib_concepts)
 
-#define __cpp_lib_forward_like   202207L
-#define __cpp_lib_invoke_r       202106L
-#define __cpp_lib_ios_noreplace  202207L
-#define __cpp_lib_is_scoped_enum 202011L
-
-#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, Clang and EDG support for modules
-#define __cpp_lib_modules 202207L
-#endif // !defined(__clang__) && !defined(__EDG__)
-
+#define __cpp_lib_forward_like       202207L
+#define __cpp_lib_invoke_r           202106L
+#define __cpp_lib_ios_noreplace      202207L
+#define __cpp_lib_is_scoped_enum     202011L
 #define __cpp_lib_move_only_function 202110L
 
 #ifdef __cpp_lib_concepts

stl/modules/std.ixx

@@ -57,7 +57,9 @@ export module std;
 #include <deque>
 #include <exception>
 #include <execution>
+#if _HAS_CXX23
 #include <expected>
+#endif // _HAS_CXX23
 #include <filesystem>
 #include <format>
 #include <forward_list>
@@ -84,7 +86,9 @@ export module std;
 #include <numeric>
 #include <optional>
 #include <ostream>
+#if _HAS_CXX23
 #include <print>
+#endif // _HAS_CXX23
 #include <queue>
 #include <random>
 #include <ranges>
@@ -96,12 +100,18 @@ export module std;
 #include <shared_mutex>
 #include <source_location>
 #include <span>
+#if _HAS_CXX23
 #include <spanstream>
+#endif // _HAS_CXX23
 #include <sstream>
 #include <stack>
+#if _HAS_CXX23
 #include <stacktrace>
+#endif // _HAS_CXX23
 #include <stdexcept>
+#if _HAS_CXX23
 #include <stdfloat>
+#endif // _HAS_CXX23
 #include <stop_token>
 #include <streambuf>
 #include <string>

tests/std/include/test_header_units_and_modules.hpp

@@ -216,12 +216,14 @@ void test_execution() {
     assert(count(execution::par, begin(arr), end(arr), 0) == 4);
 }
 
+#if TEST_STANDARD >= 23
 void test_expected() {
     using namespace std;
     puts("Testing <expected>.");
     constexpr expected<double, int> test{unexpect, 42};
     assert(test.error() == 42);
 }
+#endif // TEST_STANDARD >= 23
 
 void test_filesystem() {
     using namespace std;
@@ -492,6 +494,7 @@ void test_ostream() {
     assert(os.rdbuf() == nullptr);
 }
 
+#if TEST_STANDARD >= 23
 void test_print() {
     using namespace std;
     puts("Testing <print>.");
@@ -501,6 +504,7 @@ void test_print() {
     println(cout, "The answer to life, the universe, and everything: {}", 42);
 #endif // _CPPRTTI
 }
+#endif // TEST_STANDARD >= 23
 
 void test_queue() {
     using namespace std;
@@ -688,6 +692,7 @@ void test_span() {
     static_assert(mid[0] == 22 && mid[1] == 33 && mid[2] == 44);
 }
 
+#if TEST_STANDARD >= 23
 void test_spanstream() {
     using namespace std;
     puts("Testing <spanstream>.");
@@ -720,6 +725,7 @@ void test_spanstream() {
     s << 10 << 20 << 30;
     assert(equal(begin(s.span()), end(s.span()), begin(expected_val), end(expected_val)));
 }
+#endif // TEST_STANDARD >= 23
 
 void test_sstream() {
     using namespace std;
@@ -746,6 +752,7 @@ void test_stack() {
     assert(s.empty());
 }
 
+#if TEST_STANDARD >= 23
 __declspec(dllexport) void test_stacktrace() { // export test_stacktrace to have it named even without debug info
     using namespace std;
     puts("Testing <stacktrace>.");
@@ -761,6 +768,7 @@ __declspec(dllexport) void test_stacktrace() { // export test_stacktrace to have
 
     assert(desc == "test_stacktrace");
 }
+#endif // TEST_STANDARD >= 23
 
 void test_stdexcept() {
     using namespace std;
@@ -778,11 +786,13 @@ void test_stdexcept() {
     assert(caught_puppies);
 }
 
+#if TEST_STANDARD >= 23
 void test_stdfloat() {
     using namespace std;
     puts("Testing <stdfloat>.");
     // `namespace std` is available, so we're done.
 }
+#endif // TEST_STANDARD >= 23
 
 void test_stop_token() {
     using namespace std;
@@ -1080,7 +1090,9 @@ void all_cpp_header_tests() {
     test_deque();
     test_exception();
     test_execution();
+#if TEST_STANDARD >= 23
     test_expected();
+#endif // TEST_STANDARD >= 23
     test_filesystem();
     test_format();
     test_forward_list();
@@ -1107,7 +1119,9 @@ void all_cpp_header_tests() {
     test_numeric();
     test_optional();
     test_ostream();
+#if TEST_STANDARD >= 23
     test_print();
+#endif // TEST_STANDARD >= 23
     test_queue();
     test_random();
     test_ranges();
@@ -1119,12 +1133,18 @@ void all_cpp_header_tests() {
     test_shared_mutex();
     test_source_location();
     test_span();
+#if TEST_STANDARD >= 23
     test_spanstream();
+#endif // TEST_STANDARD >= 23
     test_sstream();
     test_stack();
+#if TEST_STANDARD >= 23
     test_stacktrace();
+#endif // TEST_STANDARD >= 23
     test_stdexcept();
+#if TEST_STANDARD >= 23
     test_stdfloat();
+#endif // TEST_STANDARD >= 23
     test_stop_token();
     test_streambuf();
     test_string();

tests/std/tests/P1502R1_standard_library_header_units/env.lst

@@ -5,7 +5,8 @@ RUNALL_INCLUDE ..\..\..\universal_prefix.lst
 RUNALL_CROSSLIST
 PM_CL="/w14365 /D_ENFORCE_FACET_SPECIALIZATIONS=1 /D_STL_CALL_ABORT_INSTEAD_OF_INVALID_PARAMETER"
 RUNALL_CROSSLIST
-PM_CL="/w14640 /Zc:threadSafeInit- /EHsc /std:c++latest"
+PM_CL="/w14640 /Zc:threadSafeInit- /EHsc /DTEST_STANDARD=20 /std:c++20"
+PM_CL="/w14640 /Zc:threadSafeInit- /EHsc /DTEST_STANDARD=23 /std:c++latest"
 RUNALL_CROSSLIST
 PM_CL="/Zc:preprocessor"
 RUNALL_CROSSLIST

tests/std/tests/P1502R1_standard_library_header_units/test.cpp

@@ -27,7 +27,9 @@ import <coroutine>;
 import <deque>;
 import <exception>;
 import <execution>;
+#if TEST_STANDARD >= 23
 import <expected>;
+#endif // TEST_STANDARD >= 23
 import <filesystem>;
 import <format>;
 import <forward_list>;
@@ -54,7 +56,9 @@ import <numbers>;
 import <numeric>;
 import <optional>;
 import <ostream>;
+#if TEST_STANDARD >= 23
 import <print>;
+#endif // TEST_STANDARD >= 23
 import <queue>;
 import <random>;
 import <ranges>;
@@ -66,12 +70,18 @@ import <set>;
 import <shared_mutex>;
 import <source_location>;
 import <span>;
+#if TEST_STANDARD >= 23
 import <spanstream>;
+#endif // TEST_STANDARD >= 23
 import <sstream>;
 import <stack>;
+#if TEST_STANDARD >= 23
 import <stacktrace>;
+#endif // TEST_STANDARD >= 23
 import <stdexcept>;
+#if TEST_STANDARD >= 23
 import <stdfloat>;
+#endif // TEST_STANDARD >= 23
 import <stop_token>;
 import <streambuf>;
 import <string>;

tests/std/tests/P2465R3_standard_library_modules/env.lst

@@ -5,7 +5,8 @@ RUNALL_INCLUDE ..\..\..\universal_prefix.lst
 RUNALL_CROSSLIST
 PM_CL="/w14365 /D_ENFORCE_FACET_SPECIALIZATIONS=1 /D_STL_CALL_ABORT_INSTEAD_OF_INVALID_PARAMETER"
 RUNALL_CROSSLIST
-PM_CL="/w14640 /Zc:threadSafeInit- /EHsc /std:c++latest"
+PM_CL="/w14640 /Zc:threadSafeInit- /EHsc /DTEST_STANDARD=20 /std:c++20"
+PM_CL="/w14640 /Zc:threadSafeInit- /EHsc /DTEST_STANDARD=23 /std:c++latest"
 RUNALL_CROSSLIST
 PM_CL="/Zc:preprocessor"
 RUNALL_CROSSLIST

tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp

@@ -1506,7 +1506,7 @@ STATIC_ASSERT(__cpp_lib_memory_resource == 201603L);
 #endif
 #endif
 
-#if _HAS_CXX23 && !defined(__clang__) && !defined(__EDG__) // TRANSITION, Clang and EDG support for modules
+#if _HAS_CXX20 && !defined(__clang__) && !defined(__EDG__) // TRANSITION, Clang and EDG support for modules
 #ifndef __cpp_lib_modules
 #error __cpp_lib_modules is not defined
 #elif __cpp_lib_modules != 202207L