Adds back special-case for booleans in std::variant gated by

_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT.

Author: Bogdan Graur

libcxx/docs/Status/Cxx20Papers.csv

@@ -173,7 +173,7 @@
 "`P1868R2 <https://wg21.link/P1868R2>`__","LWG","width: clarifying units of width and precision in std::format","Prague","|Complete|","14.0"
 "`P1937R2 <https://wg21.link/P1937R2>`__","CWG","Fixing inconsistencies between constexpr and consteval functions","Prague","* *",""
 "`P1956R1 <https://wg21.link/P1956R1>`__","LWG","On the names of low-level bit manipulation functions","Prague","|Complete|","12.0"
-"`P1957R2 <https://wg21.link/P1957R2>`__","CWG","Converting from ``T*``\  to bool should be considered narrowing (re: US 212)","Prague","|Complete|","18.0"
+"`P1957R2 <https://wg21.link/P1957R2>`__","CWG","Converting from ``T*``\  to bool should be considered narrowing (re: US 212)","Prague","* *",""
 "`P1963R0 <https://wg21.link/P1963R0>`__","LWG","Fixing US 313","Prague","* *","",""
 "`P1964R2 <https://wg21.link/P1964R2>`__","LWG","Wording for boolean-testable","Prague","|Complete|","13.0"
 "`P1970R2 <https://wg21.link/P1970R2>`__","LWG","Consistency for size() functions: Add ranges::ssize","Prague","|Complete|","15.0","|ranges|"

libcxx/include/variant

@@ -1252,6 +1252,24 @@ struct __overload {
   auto operator()(_Tp, _Up&&) const -> __check_for_narrowing<_Tp, _Up>;
 };
 
+#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 +146,13 @@ void test_T_assignment_sfinae() {
     };
     static_assert(!std::is_assignable<V, X>::value,
                   "no boolean conversion in operator=");
+#if defined(_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT)
+    static_assert(!std::is_assignable<V, std::false_type>::value,
+                  "no converted to bool in operator=");
+#else
     static_assert(std::is_assignable<V, std::false_type>::value,
                   "converted to bool in operator=");
+#endif
   }
   {
     struct X {};
@@ -296,21 +301,25 @@ void test_T_assignment_performs_assignment() {
 #endif // TEST_HAS_NO_EXCEPTIONS
 }
 
+#if !defined(_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT)
 void test_T_assignment_vector_bool() {
   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**) {
   test_T_assignment_basic();
   test_T_assignment_performs_construction();
   test_T_assignment_performs_assignment();
   test_T_assignment_noexcept();
   test_T_assignment_sfinae();
+#if !defined(_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT)
   test_T_assignment_vector_bool();
+#endif
 
   return 0;
 }

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

@@ -33,8 +33,13 @@ 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, "");
-
+#if defined(_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT)
+  static_assert(!std::is_assignable<std::variant<int, const volatile bool>, decltype("meow")>::value, "");
+  static_assert(!std::is_assignable<std::variant<bool>, std::true_type>::value, "");
+#else
   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 +80,13 @@ void test_T_ctor_sfinae() {
     };
     static_assert(!std::is_constructible<V, X>::value,
                   "no boolean conversion in constructor");
+#if defined(_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT)
+    static_assert(!std::is_constructible<V, std::false_type>::value,
+                  "no converted to bool in constructor");
+#else
     static_assert(std::is_constructible<V, std::false_type>::value,
                   "converted to bool in constructor");
+#endif
   }
   {
     struct X {};
@@ -199,19 +204,23 @@ void test_construction_with_repeated_types() {
   static_assert(std::is_constructible<V, Bar>::value, "");
 }
 
+#if !defined(_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT)
 void test_vector_bool() {
   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**) {
   test_T_ctor_basic();
   test_T_ctor_noexcept();
   test_T_ctor_sfinae();
   test_no_narrowing_check_for_class_types();
   test_construction_with_repeated_types();
+#if !defined(_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT)
   test_vector_bool();
+#endif
   return 0;
 }

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

@@ -32,8 +32,13 @@ 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, "");
-
+#if defined(_LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT)
+  static_assert(!std::is_constructible<std::variant<int, const volatile bool>, decltype("meow")>::value, "");
+  static_assert(!std::is_constructible<std::variant<bool>, std::true_type>::value, "");
+#else
   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, "");