[libc++] Implement LWG-3655: The INVOKE operation and union types.

[ldionne]

Summary

https://cplusplus.github.io/LWG/issue3655

[philnik]

libcxx/test/std/utilities/function.objects/func.invoke/invoke.pass.cpp

366

Could you also add runtime tests for this?

[ldionne]

[Github PR transition cleanup]

Commandeering to finish.

[ldionne]

Rebase and address comments.

[fsb4000]

Thank you for finishing it!

libcxx/docs/Status/Cxx23Issues.csv

 "`3857 <https://wg21.link/LWG3857>`__","``basic_string_view`` should allow explicit conversion when only traits vary","February 2023","|Complete|","17.0",""
 "`3860 <https://wg21.link/LWG3860>`__","``range_common_reference_t`` is missing","February 2023","|Complete|","17.0","|ranges|"
 "`3866 <https://wg21.link/LWG3866>`__","Bad Mandates for ``expected::transform_error`` overloads","February 2023","|Complete|","17.0",""
 "`3867 <https://wg21.link/LWG3867>`__","Should ``std::basic_osyncstream``'s move assignment operator be ``noexcept``?","February 2023","","",""
 "`3441 <https://wg21.link/LWG3441>`__","Misleading note about calls to customization points","February 2023","","",""
 "`3622 <https://wg21.link/LWG3622>`__","Misspecified transitivity of equivalence in §[unord.req.general]","February 2023","","",""
 "`3631 <https://wg21.link/LWG3631>`__","``basic_format_arg(T&&)`` should use ``remove_cvref_t<T>`` throughout","February 2023","|Complete|","17.0",""
 "`3645 <https://wg21.link/LWG3645>`__","``resize_and_overwrite`` is overspecified to call its callback with lvalues","February 2023","|Complete|","14.0",""
-"`3655 <https://wg21.link/LWG3655>`__","The ``INVOKE`` operation and union types","February 2023","","",""
+"`3655 <https://wg21.link/LWG3655>`__","The ``INVOKE`` operation and union types","February 2023","|Complete|","18.0",""
 "`3723 <https://wg21.link/LWG3723>`__","``priority_queue::push_range`` needs to ``append_range``","February 2023","","","|ranges|"
 "`3734 <https://wg21.link/LWG3734>`__","Inconsistency in ``inout_ptr`` and ``out_ptr`` for empty case","February 2023","","",""
 "`3772 <https://wg21.link/LWG3772>`__","``repeat_view``'s ``piecewise`` constructor is missing Postconditions","February 2023","","","|ranges|"
 "`3786 <https://wg21.link/LWG3786>`__","Flat maps' deduction guide needs to default ``Allocator`` to be useful","February 2023","","",""
 "`3803 <https://wg21.link/LWG3803>`__","``flat_foo`` constructors taking ``KeyContainer`` lack ``KeyCompare`` parameter","February 2023","","",""
 "`3810 <https://wg21.link/LWG3810>`__","CTAD for ``std::basic_format_args``","February 2023","|Complete|","17.0","|format|"
 "`3827 <https://wg21.link/LWG3827>`__","Deprecate ``<stdalign.h>`` and ``<stdbool.h>`` macros","February 2023","","",""
 "`3828 <https://wg21.link/LWG3828>`__","Sync ``intmax_t`` and ``uintmax_t`` with C2x","February 2023","","",""

libcxx/include/__type_traits/invoke.h

 };
 
 template <class _Fp,
           class _A0,
           class _DecayFp = __decay_t<_Fp>,
           class _DecayA0 = __decay_t<_A0>,
           class _ClassT  = typename __member_pointer_class_type<_DecayFp>::type>
 using __enable_if_bullet1 =
-    __enable_if_t<is_member_function_pointer<_DecayFp>::value && is_base_of<_ClassT, _DecayA0>::value>;
+    __enable_if_t<is_member_function_pointer<_DecayFp>::value &&
+                  (is_same<_ClassT, _DecayA0>::value || is_base_of<_ClassT, _DecayA0>::value)>;
 
 template <class _Fp, class _A0, class _DecayFp = __decay_t<_Fp>, class _DecayA0 = __decay_t<_A0> >
 using __enable_if_bullet2 =
     __enable_if_t<is_member_function_pointer<_DecayFp>::value && __is_reference_wrapper<_DecayA0>::value>;
 
 template <class _Fp,
           class _A0,
           class _DecayFp = __decay_t<_Fp>,
           class _DecayA0 = __decay_t<_A0>,
           class _ClassT  = typename __member_pointer_class_type<_DecayFp>::type>
 using __enable_if_bullet3 =
-    __enable_if_t<is_member_function_pointer<_DecayFp>::value && !is_base_of<_ClassT, _DecayA0>::value &&
+    __enable_if_t<is_member_function_pointer<_DecayFp>::value &&
+                  !(is_same<_ClassT, _DecayA0>::value || is_base_of<_ClassT, _DecayA0>::value) &&
                   !__is_reference_wrapper<_DecayA0>::value>;
 
 template <class _Fp,
           class _A0,
           class _DecayFp = __decay_t<_Fp>,
           class _DecayA0 = __decay_t<_A0>,
           class _ClassT  = typename __member_pointer_class_type<_DecayFp>::type>
 using __enable_if_bullet4 =
-    __enable_if_t<is_member_object_pointer<_DecayFp>::value && is_base_of<_ClassT, _DecayA0>::value>;
+    __enable_if_t<is_member_object_pointer<_DecayFp>::value &&
+                  (is_same<_ClassT, _DecayA0>::value || is_base_of<_ClassT, _DecayA0>::value)>;
 
 template <class _Fp, class _A0, class _DecayFp = __decay_t<_Fp>, class _DecayA0 = __decay_t<_A0> >
 using __enable_if_bullet5 =
     __enable_if_t<is_member_object_pointer<_DecayFp>::value && __is_reference_wrapper<_DecayA0>::value>;
 
 template <class _Fp,
           class _A0,
           class _DecayFp = __decay_t<_Fp>,
           class _DecayA0 = __decay_t<_A0>,
           class _ClassT  = typename __member_pointer_class_type<_DecayFp>::type>
 using __enable_if_bullet6 =
-    __enable_if_t<is_member_object_pointer<_DecayFp>::value && !is_base_of<_ClassT, _DecayA0>::value &&
+    __enable_if_t<is_member_object_pointer<_DecayFp>::value &&
+                  !(is_same<_ClassT, _DecayA0>::value || is_base_of<_ClassT, _DecayA0>::value) &&
                   !__is_reference_wrapper<_DecayA0>::value>;
 
 // __invoke forward declarations
 
 // fall back - none of the bullets
 
 template <class... _Args>
 __nat __invoke(__any, _Args&&... __args);

libcxx/test/std/utilities/function.objects/func.invoke/invoke.pass.cpp

void noexcept_test() {
         static_assert(!noexcept(std::invoke(obj)), "");
     }
     {
         MemberObj obj{42}; ((void)obj); // suppress unused warning.
         static_assert(noexcept(std::invoke(&MemberObj::x, obj)), "");
     }
 }
 
+// LWG3655: The INVOKE operation and union types
+void union_tests() {
+    union Union {
+        int x;
+        int f() { return 42; }
+        int g() const { return 52; }
+    };
+
+    // With a data member
+    {
+        auto get = []() -> Union { return Union{.x = 3}; };
+        int result = std::invoke(&Union::x, get());
+        assert(result == 3);
+    }
+    {
+        auto get = []() -> Union const { return Union{.x = 3}; };
+        int result = std::invoke(&Union::x, get());
+        assert(result == 3);
+    }
+    {
+        Union u{3};
+        int& result = std::invoke(&Union::x, u);
+        assert(&result == &u.x);
+    }
+    {
+        Union const u{3};
+        int const& result = std::invoke(&Union::x, u);
+        assert(&result == &u.x);
+    }
+
+    // With a pointer to a member function
+    {
+        auto get = []() -> Union { return Union{.x = 3}; };
+        int result = std::invoke(&Union::f, get());
+        assert(result == 42);
+    }
+    {
+        Union u{3};
+        int result = std::invoke(&Union::f, u);
+        assert(result == 42);
+    }
+    {
+        // constness mismatch
+        static_assert(!std::is_invocable_v<int (Union::*)(), Union const>);
+        static_assert(!std::is_invocable_v<int (Union::*)(), Union const&>);
+    }
+
+    {
+        auto get = []() -> Union { return Union{.x = 3}; };
+        int result = std::invoke(&Union::g, get());
+        assert(result == 52);
+    }
+    {
+        auto get = []() -> Union const { return Union{.x = 3}; };
+        int result = std::invoke(&Union::g, get());
+        assert(result == 52);
+    }
+    {
+        Union u{3};
+        int result = std::invoke(&Union::g, u);
+        assert(result == 52);
+    }
+    {
+        Union const u{3};
+        int result = std::invoke(&Union::g, u);
+        assert(result == 52);
+    }
+}
+
 int main(int, char**) {
     bullet_one_two_tests();
     bullet_three_four_tests();
     bullet_five_tests();
     noexcept_test();
 
   return 0;
 }