[PATCH] libstdc++: Optimize std::is_trivially_destructible_v

[Jonathan Wakely]

Tested x86_64-linux.

Does this look right? Can we do it faster, or simplify it?

-- >8 --

This reduces the overhead of using std::is_trivially_destructible_v and
as a result fixes some recent regressions seen with a non-default
GLIBCXX_TESTSUITE_STDS env var:
FAIL: 20_util/variant/87619.cc  -std=gnu++20 (test for excess errors)
FAIL: 20_util/variant/87619.cc  -std=gnu++23 (test for excess errors)
FAIL: 20_util/variant/87619.cc  -std=gnu++26 (test for excess errors)

libstdc++-v3/ChangeLog:

        * include/std/type_traits (is_trivially_destructible_v): Use
        built-in directly when concepts are supported.
        * testsuite/20_util/is_trivially_destructible/value_v.cc: New
        test.
---
 libstdc++-v3/include/std/type_traits          | 19 +++++++++
 .../is_trivially_destructible/value_v.cc      | 40 +++++++++++++++++++
 2 files changed, 59 insertions(+)
 create mode 100644 
libstdc++-v3/testsuite/20_util/is_trivially_destructible/value_v.cc

diff --git a/libstdc++-v3/include/std/type_traits 
b/libstdc++-v3/include/std/type_traits
index 677cd934b94..a0821347676 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -3300,9 +3300,28 @@ template <typename _Tp>
   inline constexpr bool is_trivially_move_assignable_v
     = __is_trivially_assignable(__add_lval_ref_t<_Tp>,
                                __add_rval_ref_t<_Tp>);
+
+#if __cpp_concepts
+template <typename _Tp>
+  inline constexpr bool is_trivially_destructible_v = false;
+
+template <typename _Tp>
+  requires (!is_reference_v<_Tp>) && requires (_Tp& __t) { __t.~_Tp(); }
+  inline constexpr bool is_trivially_destructible_v<_Tp>
+    = __has_trivial_destructor(_Tp);
+template <typename _Tp>
+  inline constexpr bool is_trivially_destructible_v<_Tp&> = true;
+template <typename _Tp>
+  inline constexpr bool is_trivially_destructible_v<_Tp&&> = true;
+template <typename _Tp, size_t _Nm>
+  inline constexpr bool is_trivially_destructible_v<_Tp[_Nm]>
+    = is_trivially_destructible_v<_Tp>;
+#else
 template <typename _Tp>
   inline constexpr bool is_trivially_destructible_v =
     is_trivially_destructible<_Tp>::value;
+#endif
+
 template <typename _Tp, typename... _Args>
   inline constexpr bool is_nothrow_constructible_v
     = __is_nothrow_constructible(_Tp, _Args...);
diff --git 
a/libstdc++-v3/testsuite/20_util/is_trivially_destructible/value_v.cc 
b/libstdc++-v3/testsuite/20_util/is_trivially_destructible/value_v.cc
new file mode 100644
index 00000000000..7db098a2c4d
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/is_trivially_destructible/value_v.cc
@@ -0,0 +1,40 @@
+// { dg-do compile { target c++17 } }
+#include <type_traits>
+#include <testsuite_tr1.h>
+
+template<typename T>
+constexpr void test_cv()
+{
+  static_assert(std::is_trivially_destructible_v<const T>
+      == std::is_trivially_destructible_v<T>);
+  static_assert(std::is_trivially_destructible_v<volatile T>
+      == std::is_trivially_destructible_v<T>);
+  static_assert(std::is_trivially_destructible_v<const volatile T>
+      == std::is_trivially_destructible_v<T>);
+}
+
+template<typename T, bool Expected>
+void test()
+{
+  static_assert(std::is_trivially_destructible_v<T> == Expected);
+  test_cv<T>();
+}
+
+void test01()
+{
+  using namespace __gnu_test;
+
+  test<int, true>();
+  test<int&, true>();
+  test<int&&, true>();
+  test<int[1], true>();
+  test<TType, true>();
+  test<TType[1], true>();
+  test<PODType, true>();
+  test<PODType[1], true>();
+  test<NType, false>();
+  test<SLType, false>();
+  test<int(), false>();
+  test<void, false>();
+  test<int[], false>();
+}
-- 
2.43.0