[Bug libstdc++/103240] New: std::type_info::before gives wrong answer for ARM EABI

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103240

            Bug ID: 103240
           Summary: std::type_info::before gives wrong answer for ARM EABI
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: ABI
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---
            Target: arm*-*-*gnueabi

This passes on x86_64 but fails on armv7hl:

// f1.C
#include <typeinfo>

namespace
{
  struct S { };
}

std::type_info const& t1 = typeid(S);

// f2.C
#include <typeinfo>
#include <cassert>

extern std::type_info const& t1;

namespace
{
  struct S { };
}

std::type_info const& t2 = typeid(S);

int main()
{
  if (t1 == t2)
  {
    assert( !t1.before(t2) );
    assert( !t2.before(t1) );
  }
  else
  {
    assert( t1.before(t2) || t2.before(t1) );
  }
}

$ g++ f1.C f2.C
$ ./a.out
a.out: f2.C:22: int main(): Assertion `t1.before(t2) || t2.before(t1)' failed.
qemu: uncaught target signal 6 (Aborted) - core dumped
Aborted (core dumped)

The problem is in libsupc++/tinfo2.cc

bool
type_info::before (const type_info &arg) const _GLIBCXX_NOEXCEPT
{
#if __GXX_MERGED_TYPEINFO_NAMES
  return name () < arg.name ();
#else
  return (name ()[0] == '*') ? name () < arg.name ()
    :  __builtin_strcmp (name (), arg.name ()) < 0;
#endif
}

This should have been fixed like operator== was in r179236 so that it checks
__name[0] not name()[0], otherwise the '*' is skipped and so is never found
there. That means we do a strcmp comparison of the mangled names, even when the
'*' says the types must be compared by pointer address. In the testcase the two
structs have the same mangled name, even though they are distinct types (due to
the unnamed namespaces). The string comparison therefore returns 0, and so
neither type is "before" the other, despite being distinct types.