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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107600] New: New __is_destructible built-in
Date: Thu, 10 Nov 2022 01:04:21 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: redi at gcc dot gnu.org
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107600

            Bug ID: 107600
           Summary: New __is_destructible built-in
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Like __is_construcible, __is_convertible, __is_assignable, it would be nice=
 to
have an __is_destructible built-in so we don't need to instantiate all this=
 for
every one of the C++20 concepts that depends on it:


  // In N3290 is_destructible does not say anything about function
  // types and abstract types, see LWG 2049. This implementation
  // describes function types as non-destructible and all complete
  // object types as destructible, iff the explicit destructor
  // call expression is wellformed.
  struct __do_is_destructible_impl
  {
    template<typename _Tp, typename =3D decltype(declval<_Tp&>().~_Tp())>
      static true_type __test(int);

    template<typename>
      static false_type __test(...);
  };

  template<typename _Tp>
    struct __is_destructible_impl
    : public __do_is_destructible_impl
    {
      typedef decltype(__test<_Tp>(0)) type;
    };

  template<typename _Tp,
           bool =3D __or_<is_void<_Tp>,
                        __is_array_unknown_bounds<_Tp>,
                        is_function<_Tp>>::value,
           bool =3D __or_<is_reference<_Tp>, is_scalar<_Tp>>::value>
    struct __is_destructible_safe;

  template<typename _Tp>
    struct __is_destructible_safe<_Tp, false, false>
    : public __is_destructible_impl<typename
               remove_all_extents<_Tp>::type>::type
    { };

  template<typename _Tp>
    struct __is_destructible_safe<_Tp, true, false>
    : public false_type { };

  template<typename _Tp>
    struct __is_destructible_safe<_Tp, false, true>
    : public true_type { };
  /// @endcond

  /// is_destructible
  template<typename _Tp>
    struct is_destructible
    : public __is_destructible_safe<_Tp>::type
    {
      static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
        "template argument must be a complete class or an unbounded array");
    };=