[Bug c++/126223] [15/16/17 Regression] Compiler fails with message "internal compiler error: Segmentation fault"

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126223

Kirkezz <iamkirkezz at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |iamkirkezz at gmail dot com

--- Comment #3 from Kirkezz <iamkirkezz at gmail dot com> ---
A smaller C++20 example without operator<=>.
A defaulted hidden-friend operator== on a plain aggregate
is enough, with no headers.

  // m.cppm
  export module m;

  export struct s {
      int a, b;
      friend constexpr bool operator==(s, s) = default;
  };

  // u.cpp
  import m;

  int main() { return s{1, 2} == s{1, 2} ? 0 : 1; }

  $ g++ -std=c++20 -fmodules -fmodule-mapper='|@g++-mapper-server?mods' -c
m.cppm -o m.o
  $ g++ -std=c++20 -fmodules -fmodule-mapper='|@g++-mapper-server?mods' -c
u.cpp -o u.o
  In module m, imported at u.cpp:1:
  m.cppm:5:27: internal compiler error: Segmentation fault
      5 |     friend constexpr bool operator==(s, s) = default;
        |                           ^~~~~~~~
  0x219844c internal_error(char const*, ...)
          gcc/diagnostic-global-context.cc:787
  0x102de0f crash_signal
          gcc/toplev.cc:325
  0x8bf6e1 module_state::mangle(bool)
          gcc/cp/module.cc:16692
  0x89f84c write_module
          gcc/cp/mangle.cc:1018
  0x89f84c maybe_write_module
          gcc/cp/mangle.cc:1035

Scope, all on the 16.1.0 release tarball (x86_64-pc-linux-gnu):

  friend constexpr bool operator==(s, s) = default;      ICE
  same without constexpr                                 ICE
  friend bool operator==(const s&, const s&) = default;  ICE
  friend auto operator<=>(s, s) = default;               ICE
  -std=c++20 / -std=c++23 / -std=c++26                   ICE in all three
  member form: bool operator==(const s&) const = default;  ok
  hand-written friend body                                 ok

So the trigger is: defaulted + hidden friend + declared in a class exported
from a named module + odr-used by an importer.

Given -std=c++20 reproduces the bug with the same stacktrace, the c++26
keyword may be worth dropping.