https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112591
--- Comment #9 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Tomasz Kaminski <[email protected]>: https://gcc.gnu.org/g:b3c167b61fd75fe9820636d9f7fff7920ca9b084 commit r16-6301-gb3c167b61fd75fe9820636d9f7fff7920ca9b084 Author: Tomasz KamiÅski <[email protected]> Date: Thu Dec 11 10:43:44 2025 +0100 libstdc++: Use union to store non-trivially destructible types in C++17 mode [PR112591] This patch disables use of specialization _Uninitialized<_Type, false> for non-trivially destructible types by default in C++17, and fallbacks to the primary template, that stores the type in union directly. This makes the ABI consistent between C++17 and C++20 (or later). This partial specialization is no longer required after the changes introduced in r16-5961-g09bece00d0ec98. This fixes non-conformance in C++17 mode where global variables of a variant specialization type, were not statically-initialized for non-trivially destructible types, even if initialization of the selected alternative could be performed at compile time. For illustration, the following global variable will be statically initialized after this change: std::variant<std::unique_ptr<T>, std::unique_ptr<U>> ptr; This constitutes an ABI break, and changes the layout of the types, that uses the same non-trivially copyable both as the base class, as alternative of the variant object that is first member: struct EmptyNonTrivial { ~EmptyNonTrivial(); }; struct Affected : EmptyNonTrivial { std::variant<EmptyNonTrivial, char> mem; // mem was at offset zero, // will use non-zero offset now }; After changes the layout of such types consistent with one used for empty types with trivial destructor, or one used for any empty type in C++20 or later. For programs affected by this change, it can be reverted in C++17 mode, by defining _GLIBCXX_USE_VARIANT_CXX17_OLD_ABI. However, presence of this macro has no effect in C++20 or later modes. PR libstdc++/112591 libstdc++-v3/ChangeLog: * include/std/variant (_Uninitialized::_M_get, __get_n) (_Uninitialized<_Type, false>): Add _GLIBCXX_USE_VARIANT_CXX17_OLD_ABI check to preprocessor guard. * testsuite/20_util/variant/112591.cc: Updated tests. * testsuite/20_util/variant/112591_compat.cc: New test. * testsuite/20_util/variant/constinit.cc: New test. * testsuite/20_util/variant/constinit_compat.cc: New test. Reviewed-by: Jonathan Wakely <[email protected]> Signed-off-by: Tomasz KamiÅski <[email protected]>
