[gcc r13-3870] libstdc++: Optimize std::destructible concept

[Jonathan Wakely]

https://gcc.gnu.org/g:0cbb756fe9c8e13a743bdcf599481b31bd01ddbc

commit r13-3870-g0cbb756fe9c8e13a743bdcf599481b31bd01ddbc
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Nov 10 01:30:45 2022 +0000

    libstdc++: Optimize std::destructible concept
    
    This uses variable templates and constraints to define a much simpler
    std::destructible concept. This avoids instantiating the trait
    std::is_nothrow_destructible and all its implementation in terms of
    __is_destructible_safe and __is_destructible_impl.
    
    If we had an intrinsic we could just use that (PR c++/107600).
    
    libstdc++-v3/ChangeLog:
    
            * include/std/concepts (__detail::__destructible_impl)
            (__detail::__destructible): New variable templates.
            (destructible): Use __detail::__destructible.
            * testsuite/std/concepts/concepts.lang/concept.destructible/1.cc:
            Add more checks for array and reference types.

Diff:
---
 libstdc++-v3/include/std/concepts                  | 24 ++++++++++++++++++----
 .../concepts.lang/concept.destructible/1.cc        |  5 +++++
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/std/concepts b/libstdc++-v3/include/std/concepts
index 32139063936..c062d62d24f 100644
--- a/libstdc++-v3/include/std/concepts
+++ b/libstdc++-v3/include/std/concepts
@@ -113,9 +113,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     template<typename _Tp>
       using __cref = const remove_reference_t<_Tp>&;
 
-      template<typename _Tp>
-	concept __class_or_enum
-	  = is_class_v<_Tp> || is_union_v<_Tp> || is_enum_v<_Tp>;
+    template<typename _Tp>
+      concept __class_or_enum
+	= is_class_v<_Tp> || is_union_v<_Tp> || is_enum_v<_Tp>;
+
+    template<typename _Tp>
+      constexpr bool __destructible_impl = false;
+    template<typename _Tp>
+      requires requires(_Tp& __t) { { __t.~_Tp() } noexcept; }
+      constexpr bool __destructible_impl<_Tp> = true;
+
+    template<typename _Tp>
+      constexpr bool __destructible = __destructible_impl<_Tp>;
+    template<typename _Tp>
+      constexpr bool __destructible<_Tp&> = true;
+    template<typename _Tp>
+      constexpr bool __destructible<_Tp&&> = true;
+    template<typename _Tp, size_t _Nm>
+      constexpr bool __destructible<_Tp[_Nm]> = __destructible<_Tp>;
+
   } // namespace __detail
 
   /// [concept.assignable], concept assignable_from
@@ -129,7 +145,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   /// [concept.destructible], concept destructible
   template<typename _Tp>
-    concept destructible = is_nothrow_destructible_v<_Tp>;
+    concept destructible = __detail::__destructible<_Tp>;
 
   /// [concept.constructible], concept constructible_from
   template<typename _Tp, typename... _Args>
diff --git a/libstdc++-v3/testsuite/std/concepts/concepts.lang/concept.destructible/1.cc b/libstdc++-v3/testsuite/std/concepts/concepts.lang/concept.destructible/1.cc
index a0bd0364d1b..81d72abbdaa 100644
--- a/libstdc++-v3/testsuite/std/concepts/concepts.lang/concept.destructible/1.cc
+++ b/libstdc++-v3/testsuite/std/concepts/concepts.lang/concept.destructible/1.cc
@@ -29,6 +29,7 @@ static_assert( std::destructible<int&&> );
 static_assert( std::destructible<const int&> );
 static_assert( !std::destructible<int[]> );
 static_assert( std::destructible<int[2]> );
+static_assert( std::destructible<int[2][3]> );
 static_assert( !std::destructible<int()> );
 static_assert( std::destructible<int(*)()> );
 static_assert( std::destructible<int(&)()> );
@@ -47,6 +48,8 @@ struct C
   ~C() noexcept(false) { }
 };
 static_assert( !std::destructible<C> );
+static_assert( std::destructible<C&> );
+static_assert( !std::destructible<C[1]> );
 class D
 {
 public:
@@ -55,3 +58,5 @@ private:
   ~D() { }
 };
 static_assert( !std::destructible<D> );
+static_assert( std::destructible<D&> );
+static_assert( !std::destructible<D[1]> );