See <https://wg21.link/P1967R14>.
This has been implemented already earlier in GCC 15, first as an extension based on the C23 version, so e.g. r15-3599-geba6d2aa71a9b59386e5a2453cbe924371626b0b r15-4375-g1844a4aa6615c2252303e70d41bdb18e7c5664c6 r15-5958-g0223119f1a6351543c6e96a9735e05cbd4583889 (various bugfixes as well) and today no longer pedwarns for C++26 and has the FTM defined r15-7755-gb510c53b18b7255bc43de1c45b700e0206ca27f0