613f8ddbe3
This commit implements the proposed resolution to LWG4169, which is to constrain std::atomic<T>'s default constructor based on whether T itself is default constructible. At the moment, std::atomic<T>'s primary template in libstdc++ has a defaulted default constructor. Value-initialization of the T member (since C++20 / P0883R2) is done via a NSDMI (= T()). GCC already considers the defaulted constructor constrained/deleted, however this behavior is non-standard (see the discussion in PR116769): the presence of a NSDMI should not make the constructor unavailable to overload resolution/deleted ([class.default.ctor]/2.5 does not apply). When using libstdc++ on Clang, this causes build issues as the constructor is *not* deleted there -- the interpretation of [class.default.ctor]/4 seems to match Clang's behavior. Therefore, although there would be "nothing to do" with GCC+libstdc++, this commit changes the code as to stop relying on the GCC language extension. In C++ >= 20 modes, std::atomic's defaulted default constructor is changed to be a non-defaulted one, with a constraint added as per LWG4169; value-initialization of the data member is moved from the NSDMI to the member init list. The new signature matches the one in the Standard as per [atomics.types.operations]/1. In pre-C++20 modes, the constructor is left defaulted. This ensures compatibility with C++11/14/17 behavior. In other words: we are not backporting P0883R2 to earlier language modes here. Amend an existing test to check that a std::atomic wrapping a non-default constructible type is always non-default constructible: from C++20, because of the constraint; before C++20, because we are removing the NSDMI, and therefore [class.default.ctor]/2.5 applies. Add another test that checks that std::atomic is trivially default constructible in pre-C++20 modes, and it isn't afterwards. libstdc++-v3/ChangeLog: * include/bits/version.def (atomic_value_initialization): Guard the FTM with the language concepts FTM. * include/bits/version.h: Regenerate. * include/std/atomic (atomic): When atomic value init is defined, change the defaulted default constructor to a non-defaulted one, constraining it as per LWG4169. Otherwise, keep the existing constructor. Remove the NSDMI for the _M_i member. (_GLIBCXX20_INIT): Drop the macro, as it is not needed any more. * testsuite/29_atomics/atomic/69301.cc: Test that an atomic wrapping a non-default-constructible type is always itself non-default-constructible (in all language modes). * testsuite/29_atomics/atomic/cons/trivial.cc: New test. |
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