[libc++] Re-introduce special support for narrowing conversions to bool in variant (#73121)

This patch re-introduces special support for narrowing conversions to
bool
in std::variant, which was removed in 170810f in order to make
libc++
Standards-conforming.

The special support is gated by the
`_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT`
macro and will be supported for LLVM 18 only as a courtesy to help large
code bases migrate over to the Standard behavior.

---------

Co-authored-by: Bogdan Graur <[email protected]>
Co-authored-by: Louis Dionne <[email protected]>

Author: 3 people

libcxx/docs/ReleaseNotes/18.rst

@@ -81,6 +81,12 @@ Deprecations and Removals
 - The non-conforming constructor ``std::future_error(std::error_code)`` has been removed. Please use the
   ``std::future_error(std::future_errc)`` constructor provided in C++17 instead.
 
+- `P1957 <https://wg21.link/P1957>` has been implemented in Clang and libc++ removed a code path that led to
+  narrowing conversions in ``std::variant`` behaving in a non-standard way. This may change how some uses of
+  ``std::variant``'s constructor behave in user code. The ``_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT``
+  macro is provided to restore the previous behavior, and it will be supported in the LLVM 18 release only.
+  In LLVM 19 and beyond, ``_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT`` will not be honored anymore.
+
 Upcoming Deprecations and Removals
 ----------------------------------
 

libcxx/include/variant

@@ -1252,6 +1252,25 @@ struct __overload {
   auto operator()(_Tp, _Up&&) const -> __check_for_narrowing<_Tp, _Up>;
 };
 
+// TODO(LLVM-19): Remove all occurrences of this macro.
+#ifdef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
+template <class _Tp, size_t>
+struct __overload_bool  {
+  template <class _Up, class _Ap = __remove_cvref_t<_Up>>
+  auto operator()(bool, _Up&&) const
+      -> enable_if_t<is_same_v<_Ap, bool>, __type_identity<_Tp>>;
+};
+
+template <size_t _Idx>
+struct __overload<bool, _Idx> : __overload_bool<bool, _Idx> {};
+template <size_t _Idx>
+struct __overload<bool const, _Idx> : __overload_bool<bool const, _Idx> {};
+template <size_t _Idx>
+struct __overload<bool volatile, _Idx> : __overload_bool<bool volatile, _Idx> {};
+template <size_t _Idx>
+struct __overload<bool const volatile, _Idx> : __overload_bool<bool const volatile, _Idx> {};
+#endif
+
 template <class ..._Bases>
 struct __all_overloads : _Bases... {
   void operator()() const;

libcxx/test/std/utilities/variant/variant.variant/variant.assign/T.pass.cpp

@@ -146,8 +146,10 @@ void test_T_assignment_sfinae() {
     };
     static_assert(!std::is_assignable<V, X>::value,
                   "no boolean conversion in operator=");
+#ifndef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
     static_assert(std::is_assignable<V, std::false_type>::value,
                   "converted to bool in operator=");
+#endif
   }
   {
     struct X {};
@@ -297,11 +299,13 @@ void test_T_assignment_performs_assignment() {
 }
 
 void test_T_assignment_vector_bool() {
+#ifndef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
   std::vector<bool> vec = {true};
   std::variant<bool, int> v;
   v = vec[0];
   assert(v.index() == 0);
   assert(std::get<0>(v) == true);
+#endif
 }
 
 int main(int, char**) {

libcxx/test/std/utilities/variant/variant.variant/variant.assign/conv.pass.cpp

@@ -34,7 +34,9 @@ int main(int, char**)
   static_assert(!std::is_assignable<std::variant<int, bool>, decltype("meow")>::value, "");
   static_assert(!std::is_assignable<std::variant<int, const bool>, decltype("meow")>::value, "");
 
+#ifndef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
   static_assert(std::is_assignable<std::variant<bool>, std::true_type>::value, "");
+#endif
   static_assert(!std::is_assignable<std::variant<bool>, std::unique_ptr<char> >::value, "");
   static_assert(!std::is_assignable<std::variant<bool>, decltype(nullptr)>::value, "");
 

libcxx/test/std/utilities/variant/variant.variant/variant.ctor/T.pass.cpp

@@ -80,8 +80,10 @@ void test_T_ctor_sfinae() {
     };
     static_assert(!std::is_constructible<V, X>::value,
                   "no boolean conversion in constructor");
+#ifndef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
     static_assert(std::is_constructible<V, std::false_type>::value,
                   "converted to bool in constructor");
+#endif
   }
   {
     struct X {};
@@ -200,10 +202,12 @@ void test_construction_with_repeated_types() {
 }
 
 void test_vector_bool() {
+#ifndef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
   std::vector<bool> vec = {true};
   std::variant<bool, int> v = vec[0];
   assert(v.index() == 0);
   assert(std::get<0>(v) == true);
+#endif
 }
 
 int main(int, char**) {

libcxx/test/std/utilities/variant/variant.variant/variant.ctor/conv.pass.cpp

@@ -33,7 +33,9 @@ int main(int, char**)
   static_assert(!std::is_constructible<std::variant<int, bool>, decltype("meow")>::value, "");
   static_assert(!std::is_constructible<std::variant<int, const bool>, decltype("meow")>::value, "");
 
+#ifndef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
   static_assert(std::is_constructible<std::variant<bool>, std::true_type>::value, "");
+#endif
   static_assert(!std::is_constructible<std::variant<bool>, std::unique_ptr<char> >::value, "");
   static_assert(!std::is_constructible<std::variant<bool>, decltype(nullptr)>::value, "");
 

libcxx/test/support/variant_test_helpers.h

@@ -23,10 +23,11 @@
 
 // FIXME: Currently the variant<T&> tests are disabled using this macro.
 #define TEST_VARIANT_HAS_NO_REFERENCES
+
+// TODO(LLVM-19): Remove TEST_VARIANT_ALLOWS_NARROWING_CONVERSIONS
 #ifdef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
 # define TEST_VARIANT_ALLOWS_NARROWING_CONVERSIONS
 #endif
-
 #ifdef TEST_VARIANT_ALLOWS_NARROWING_CONVERSIONS
 constexpr bool VariantAllowsNarrowingConversions = true;
 #else