Add release note, simplify tests.

Author: ldionne

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 +1252,7 @@ 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  {

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

@@ -146,10 +146,7 @@ void test_T_assignment_sfinae() {
     };
     static_assert(!std::is_assignable<V, X>::value,
                   "no boolean conversion in operator=");
-#ifdef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
-    static_assert(!std::is_assignable<V, std::false_type>::value,
-                  "no converted to bool in operator=");
-#else
+#ifndef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
     static_assert(std::is_assignable<V, std::false_type>::value,
                   "converted to bool in operator=");
 #endif

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

@@ -33,13 +33,10 @@ 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, "");
-#ifdef _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
+
+#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

@@ -80,10 +80,7 @@ void test_T_ctor_sfinae() {
     };
     static_assert(!std::is_constructible<V, X>::value,
                   "no boolean conversion in constructor");
-#ifdef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
-    static_assert(!std::is_constructible<V, std::false_type>::value,
-                  "no converted to bool in constructor");
-#else
+#ifndef _LIBCPP_ENABLE_NARROWING_CONVERSIONS_IN_VARIANT
     static_assert(std::is_constructible<V, std::false_type>::value,
                   "converted to bool in constructor");
 #endif

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

@@ -32,13 +32,10 @@ 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, "");
-#ifdef _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
+
+#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