https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99375
--- Comment #2 from Arfrever Frehtes Taifersar Arahesis <Arfrever.FTA at GMail dot Com> --- I confirm that backporting of GCC commit 1ab88985631dd2c5a5e3b5c0dce47cf8b6ed2f82 (https://gcc.gnu.org/g:1ab88985631dd2c5a5e3b5c0dce47cf8b6ed2f82) to GCC 10.2.0 fixes this problem (also with plain CXXFLAGS="-O3").
