https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102357
--- Comment #2 from jim x <xmh970252187 at gmail dot com> --- This part in c++20 is more clear than c++17, which is added since c++20, see https://timsong-cpp.github.io/cppwp/n4861/dcl.fct.def.default#2. What the version I'm testing the code is GCC 12.0 with -std=c++20 command. See https://godbolt.org/z/d7h4Mj3qG
