[C++ PATCH, RFC] Implement new C++ intrinsics __is_assignable and __is_constructible.

[Ville Voutilainen <ville dot voutilainen at gmail dot com>]

I have tested this with the full suite on Linux-PPC64. It works otherwise fine,
but there's one snag: 20_util/unique_ptr/specialized_algorithms/swap_cxx17.cc
fails, and it looks like the trait ends up instantiating the definition
of a destructor, which then ends up being hard-error ill-formed.

That is, we have

struct C { };
void swap(C&, C&) = delete;
static_assert( !std::is_swappable_v<std::unique_ptr<int, C>> );

struct D { D(D&&) = delete; };
static_assert( !std::is_swappable_v<std::unique_ptr<int, D>> );

The destructor definitions of unique_ptr<int, C> and unique_ptr<int D>
are not valid because that destructor will try to call an operator()
on those deleters, and they don't have that operator.

The existing library implementation of is_constructible doesn't
instantiate the destructor definitions, or if it does, they don't cause
a hard error. This intrinsic trait approach behaves differently, and I would
welcome any help fixing that problem.

2017-05-12  Ville Voutilainen  <ville.voutilainen@gmail.com>

    c-family/

    Implement new C++ intrinsics __is_assignable and __is_constructible.
    * c-common.c (__is_assignable, __is_constructible): New.
    * c-common.h (RID_IS_ASSIGNABLE, RID_IS_CONSTRUCTIBLE): Likewise.

    cp/

    Implement new C++ intrinsics __is_assignable and __is_constructible.
    * cp-tree.h (CPTK_IS_ASSIGNABLE, CPTK_IS_CONSTRUCTIBLE): New.
    (is_xible): New.
    * cxx-pretty-print.c (pp_cxx_trait_expression): Handle
    CPTK_IS_ASSIGNABLE and CPTK_IS_CONSTRUCTIBLE.
    * method.c (is_xible_helper): New.
    (is_trivially_xible): Adjust.
    (is_xible): New.
    * parser.c (cp_parser_primary_expression): Handle
    RID_IS_ASSIGNABLE and RID_IS_CONSTRUCTIBLE.
    (cp_parser_trait_expr): Likewise.
    * semantics.c (trait_expr_value): Handle
    CPTK_IS_ASSIGNABLE and CPTK_IS_CONSTRUCTIBLE.

    libstdc++-v3/

    Implement new C++ intrinsics __is_assignable and __is_constructible.
    * include/std/type_traits (__do_is_static_castable_impl): Remove.
    (__is_static_castable_impl, __is_static_castable_safe): Likewise.
    (__is_static_castable, __do_is_direct_constructible_impl): Likewise.
    (__is_direct_constructible_impl): Likewise.
    (__is_direct_constructible_new_safe): Likewise.
    (__is_base_to_derived_ref, __is_lvalue_to_rvalue_ref): Likewise.
    (__is_direct_constructible_ref_cast): Likewise.
    (__is_direct_constructible_new, __is_direct_constructible): Likewise.
    (__do_is_nary_constructible_impl): Likewise.
    (__is_nary_constructible_impl, __is_nary_constructible): Likewise.
    (__is_constructible_impl): Likewise.
    (is_constructible): Call the intrinsic.
    (__is_assignable_helper): Remove.
    (is_assignable): Call the intrinsic.
    (is_trivially_constructible): Likewise.
    (is_trivially_assignable): Likewise.
    (testsuite/20_util/declval/requirements/1_neg.cc): Adjust.
    (testsuite/20_util/make_signed/requirements/typedefs_neg.cc): Likewise.
    (testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc):
    Likewise.

Attachment: is_constructible.diff
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