[Bug libstdc++/68739] FAIL: 30_threads/call_once/constexpr.cc (test for excess errors)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68739

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The standard says those types must have a constexpr constructor, but we can't
implement that on all targets:

  // Common base class for std::mutex and std::timed_mutex
  class __mutex_base
  {
  protected:
    typedef __gthread_mutex_t                   __native_type;

#ifdef __GTHREAD_MUTEX_INIT
    __native_type  _M_mutex = __GTHREAD_MUTEX_INIT;

    constexpr __mutex_base() noexcept = default;
#else
    __native_type  _M_mutex;

    __mutex_base() noexcept
    {
      // XXX EAGAIN, ENOMEM, EPERM, EBUSY(may), EINVAL(may)
      __GTHREAD_MUTEX_INIT_FUNCTION(&_M_mutex);
    }

    ~__mutex_base() noexcept { __gthread_mutex_destroy(&_M_mutex); }
#endif

...

  class mutex : private __mutex_base
  {
  public:
    typedef __native_type*                      native_handle_type;

#ifdef __GTHREAD_MUTEX_INIT
   constexpr
#endif
   mutex() noexcept = default;



I think we simply need to disable the tests for targets that don't support
static initializers such as PTHREAD_MUTEX_INITIALIZER.