[Bug c++/123879] [16 Regression] -std=c++26 --compile-std-module -fmodules fails with installed GCC

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123879

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
With GCC 15 this works:

g++ -fmodules -fmodule-only -std=c++26 -c -fsearch-include-path \
   bits/std.cc bits/std.compat.cc

With GCC 16 the same command fails with the errors in comment 0.

So --compile-std-module isn't needed to reproduce it.

P2 seems reasonable though, as it does require C++26 (the stdbit.h header is
empty for C++20 and C++23).