[Bug libstdc++/100676] New: Redeclaring __failed_assertion() at every point of use of __glibcxx_assert breaks Clang CUDA

["joachim at joameyer dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100676

            Bug ID: 100676
           Summary: Redeclaring __failed_assertion() at every point of use
                    of __glibcxx_assert breaks Clang CUDA
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: joachim at joameyer dot de
  Target Milestone: ---

As noted in https://bugs.llvm.org/show_bug.cgi?id=50383 the use of
__glibcxx_assert in the complex functions breaks using the complex header with
Clang CUDA. The way Clang CUDA handles that might not be ideal, but the
libstdc++ issue seems to be present anyways:

In every use of __glibcxx_assert the function __failed_assertion is redeclared
and btw. never defined, which might break other projects as well, and then is
used as an assertion in constant evaluated contexts, as it is a non constexpr
function.

I wasn't able to come up with an entirely better approach so far, but I guess
most of the issues, that I could think of so far, would be solved by moving the
void __failed_assertion(); declaration out of the macro.

This would fix the issues with Clang CUDA for example:
https://cuda.godbolt.org/z/GTMnxez7j