libstdc++: Implement P1972R2 changes to std::variant (PR 95832)

G++ implements P1972R2 since r11-1597-0ca22d027ecc and so we no longer
need the P0608R3 special case to prevent narrowing conversions to bool.

Since non-GNU compilers don't necessarily implment P1972R2 yet, this
may cause a regression for those compilers. There is no feature-test
macro we can use to detect it though, so we'll have to live with it.

libstdc++-v3/ChangeLog:

	PR libstdc++/95832
	* include/std/variant (__detail::__variant::_Build_FUN): Remove
	partial specialization to prevent narrowing conversions to bool.
	* testsuite/20_util/variant/compile.cc: Test non-narrowing
	conversions to bool.
	* testsuite/20_util/variant/run.cc: Likewise.

libstdc++-v3/include/std/variant

@@ -164,7 +164,7 @@ namespace __detail
 {
 namespace __variant
 {
-  // Returns the first appearence of _Tp in _Types.
+  // Returns the first appearance of _Tp in _Types.
   // Returns sizeof...(_Types) if _Tp is not in _Types.
   template<typename _Tp, typename... _Types>
     struct __index_of : std::integral_constant<size_t, 0> {};
@@ -733,10 +733,8 @@ namespace __variant
   // Helper used to check for valid conversions that don't involve narrowing.
   template<typename _Ti> struct _Arr { _Ti _M_x[1]; };
 
-  // Build an imaginary function FUN(Ti) for each alternative type Ti
-  template<size_t _Ind, typename _Tp, typename _Ti,
-	   bool _Ti_is_cv_bool = is_same_v<remove_cv_t<_Ti>, bool>,
-	   typename = void>
+  // "Build an imaginary function FUN(Ti) for each alternative type Ti"
+  template<size_t _Ind, typename _Tp, typename _Ti, typename = void>
     struct _Build_FUN
     {
       // This function means 'using _Build_FUN<I, T, Ti>::_S_fun;' is valid,
@@ -744,24 +742,15 @@ namespace __variant
       void _S_fun();
     };
 
-  // ... for which Ti x[] = {std::forward<T>(t)}; is well-formed,
+  // "... for which Ti x[] = {std::forward<T>(t)}; is well-formed."
   template<size_t _Ind, typename _Tp, typename _Ti>
-    struct _Build_FUN<_Ind, _Tp, _Ti, false,
+    struct _Build_FUN<_Ind, _Tp, _Ti,
 		      void_t<decltype(_Arr<_Ti>{{std::declval<_Tp>()}})>>
     {
       // This is the FUN function for type _Ti, with index _Ind
       static integral_constant<size_t, _Ind> _S_fun(_Ti);
     };
 
-  // ... and if Ti is cv bool, remove_cvref_t<T> is bool.
-  template<size_t _Ind, typename _Tp, typename _Ti>
-    struct _Build_FUN<_Ind, _Tp, _Ti, true,
-		      enable_if_t<is_same_v<__remove_cvref_t<_Tp>, bool>>>
-    {
-      // This is the FUN function for when _Ti is cv bool, with index _Ind
-      static integral_constant<size_t, _Ind> _S_fun(_Ti);
-    };
-
   template<typename _Tp, typename _Variant,
 	   typename = make_index_sequence<variant_size_v<_Variant>>>
     struct _Build_FUNs;

libstdc++-v3/testsuite/20_util/variant/compile.cc

@@ -155,6 +155,14 @@ void arbitrary_ctor()
   static_assert(!is_constructible_v<variant<int>, unsigned>);
   static_assert(!is_constructible_v<variant<bool>, int>);
   static_assert(!is_constructible_v<variant<bool>, void*>);
+
+  // P1957R2 Converting from T* to bool should be considered narrowing
+  struct ConvertibleToBool
+  {
+    operator bool() const { return true; }
+  };
+  static_assert(is_constructible_v<variant<bool>, ConvertibleToBool>);
+  static_assert(is_constructible_v<variant<bool, int>, ConvertibleToBool>);
 }
 
 struct none { none() = delete; };

libstdc++-v3/testsuite/20_util/variant/run.cc

@@ -139,6 +139,20 @@ void arbitrary_ctor()
     variant<float, big_int> v3 = 0;
     VERIFY(v3.index() == 1);
   }
+
+  {
+    // P1957R2 Converting from T* to bool should be considered narrowing
+    struct ConvertibleToBool
+    {
+      operator bool() const { return true; }
+    };
+    variant<bool> v1 = ConvertibleToBool();
+    VERIFY(std::get<0>(v1) == true);
+    variant<bool, int> v2 = ConvertibleToBool();
+    VERIFY(std::get<0>(v2) == true);
+    variant<int, bool> v3 = ConvertibleToBool();
+    VERIFY(std::get<1>(v3) == true);
+  }
 }
 
 struct ThrowingMoveCtorThrowsCopyCtor
@@ -226,6 +240,23 @@ void arbitrary_assign()
     v3 = 0;
     VERIFY(v3.index() == 1);
   }
+
+  {
+    // P1957R2 Converting from T* to bool should be considered narrowing
+    struct ConvertibleToBool
+    {
+      operator bool() const { return true; }
+    };
+    variant<bool> v1;
+    v1 = ConvertibleToBool();
+    VERIFY(std::get<0>(v1) == true);
+    variant<bool, int> v2;
+    v2 = ConvertibleToBool();
+    VERIFY(std::get<0>(v2) == true);
+    variant<int, bool> v3;
+    v3 = ConvertibleToBool();
+    VERIFY(std::get<1>(v3) == true);
+  }
 }
 
 void dtor()