[Bug c++/127050] [modules] hidden friend declaration in exported class cannot resolve to unexported free function definition

[dr.xiaosa at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127050

--- Comment #2 from dr.xiaosa at gmail dot com ---
(In reply to Patrick Palka from comment #1)
> Thanks for the bug report and reduced testcase. Was it reduced from a
> <ranges> example?  If so, it'd be interesting to see a <ranges>-based
> testcase too.

It actually comes from a report of a Clang C++ modules bug:

https://github.com/llvm/llvm-project/issues/218228

Original testcase works on GCC but fails on Clang with libstdc++ std module. I
managed to reduce it to be independent from any std module per requested,
however the reduced testcase starts to fail on GCC with a different error. Then
I take another round to figure out whether the testcase can compile on GCC
again if I add something, and finally reach here.