[Bug libstdc++/85222] [7/8 Regression] ABI breakage of __throw_ios_failure by r244498

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85222

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Seems simpler to just define:

struct __dual_ios_failure {
  __dual_ios_failure(std::string s, error_code e) : new_(s, e), old_(s) { }
  ios::failure[abi:cxx11] new_;
  ios::failure old_;
};

and make __throw_ios_failure() throw one of that type, and make the EH runtime
do the necessary adjustments to make this work:

__dual_ios_failure * p1;

try {
  try {
    try {
      throw __dual_ios_failure("", {});
    } catch (__dual_ios_failure& e1) {
      p1 = &e1;
      throw;
    }
  } catch (ios::failure[abi:cxx11]& e2) {
    assert( &e2 == &p1->new_ );
    throw;
  }
} catch (ios::failure& e3) {
  assert( &e3 == &p1->old_ );
}

i.e. if the catch handler is one of the ios::failure types and the actual
thrown exception is __dual_ios_failure then catch the member instead of the
object itself. The "throw;" would re-throw the original object of type
__dual_abi_failure, so the next handler would be able to perform the same
checks and adjustments.

This would only require magic in the EH catch routines, not a new way to
declare base classes.