align is_replaceable with p2786 semantics and allow user opt-in

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

Author: guptapratykshh

libs/core/type_support/include/hpx/type_support/is_replaceable.hpp

@@ -14,14 +14,36 @@
 
 namespace hpx::experimental {
 
-    HPX_CXX_EXPORT template <typename T>
+#if defined(__cpp_lib_trivially_relocatable)
+    HPX_CXX_CORE_EXPORT template <typename T>
+    struct is_replaceable : std::is_replaceable<T>
+    {
+    };
+#else
+    HPX_CXX_CORE_EXPORT template <typename T>
     struct is_replaceable
-      : std::bool_constant<std::is_move_constructible_v<T> &&
-            std::is_move_assignable_v<T> && is_trivially_relocatable_v<T>>
+      : std::bool_constant<std::is_object_v<T> && !std::is_const_v<T> &&
+            !std::is_volatile_v<T> &&
+            (std::is_scalar_v<T> ||
+                ((std::is_class_v<T> || std::is_union_v<T>) &&
+                    std::is_trivially_move_constructible_v<T> &&
+                    std::is_trivially_move_assignable_v<T> &&
+                    std::is_trivially_destructible_v<T>) )>
+    {
+    };
+
+    HPX_CXX_CORE_EXPORT template <typename T>
+    struct is_replaceable<T[]> : std::false_type
+    {
+    };
+
+    HPX_CXX_CORE_EXPORT template <typename T, std::size_t N>
+    struct is_replaceable<T[N]> : std::false_type
     {
     };
+#endif
 
-    HPX_CXX_EXPORT template <typename T>
+    HPX_CXX_CORE_EXPORT template <typename T>
     inline constexpr bool is_replaceable_v = is_replaceable<T>::value;
 
 }    // namespace hpx::experimental

libs/core/type_support/tests/unit/is_replaceable.cpp

@@ -42,9 +42,14 @@ static_assert(!is_replaceable_v<void>);
 // std::mutex is not move assignable, nor trivially relocatable
 static_assert(!is_replaceable_v<std::mutex>);
 
+struct trivial_class
+{
+    int x;
+};
+static_assert(is_replaceable_v<trivial_class>);
+
 struct not_destructible
 {
-    not_destructible(not_destructible const&);
     not_destructible(not_destructible&&);
     ~not_destructible() = delete;
 };
@@ -63,15 +68,16 @@ struct not_move_constructible
 };
 static_assert(!is_replaceable_v<not_move_constructible>);
 
-struct move_assignable_but_not_trivially_relocatable
+struct move_assignable_but_not_implicitly_replaceable
 {
     std::unique_ptr<int> p;
-    move_assignable_but_not_trivially_relocatable(
-        move_assignable_but_not_trivially_relocatable&&) = default;
-    move_assignable_but_not_trivially_relocatable& operator=(
-        move_assignable_but_not_trivially_relocatable&&) = default;
+    move_assignable_but_not_implicitly_replaceable(
+        move_assignable_but_not_implicitly_replaceable&&) = default;
+    move_assignable_but_not_implicitly_replaceable& operator=(
+        move_assignable_but_not_implicitly_replaceable&&) = default;
 };
-static_assert(!is_replaceable_v<move_assignable_but_not_trivially_relocatable>);
+static_assert(
+    !is_replaceable_v<move_assignable_but_not_implicitly_replaceable>);
 
 // Opt-in example
 struct opt_in_replaceable