P2166R1 Prohibiting basic_string And basic_string_view Construction From nullptr (#1995)

Co-authored-by: Casey Carter <cacarter@microsoft.com>
Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/xstring

@@ -1270,6 +1270,10 @@ public:
     /* implicit */ constexpr basic_string_view(_In_z_ const const_pointer _Ntcts) noexcept // strengthened
         : _Mydata(_Ntcts), _Mysize(_Traits::length(_Ntcts)) {}
 
+#if _HAS_CXX23
+    basic_string_view(nullptr_t) = delete;
+#endif // _HAS_CXX23
+
     constexpr basic_string_view(
         _In_reads_(_Count) const const_pointer _Cts, const size_type _Count) noexcept // strengthened
         : _Mydata(_Cts), _Mysize(_Count) {
@@ -2536,6 +2540,10 @@ public:
         _Proxy._Release();
     }
 
+#if _HAS_CXX23
+    basic_string(nullptr_t) = delete;
+#endif // _HAS_CXX23
+
 #if _HAS_CXX17
     template <class _Alloc2 = _Alloc, enable_if_t<_Is_allocator<_Alloc2>::value, int> = 0>
 #endif // _HAS_CXX17
@@ -3106,6 +3114,10 @@ public:
         return assign(_Ptr);
     }
 
+#if _HAS_CXX23
+    basic_string& operator=(nullptr_t) = delete;
+#endif // _HAS_CXX23
+
     _CONSTEXPR20_CONTAINER basic_string& operator=(const _Elem _Ch) { // assign {_Ch, _Elem()}
         _Mypair._Myval2._Mysize = 1;
         _Elem* const _Ptr       = _Mypair._Myval2._Myptr();

stl/inc/yvals_core.h

@@ -262,6 +262,7 @@
 // P1048R1 is_scoped_enum
 // P1679R3 contains() For basic_string/basic_string_view
 // P1682R3 to_underlying() For Enumerations
+// P2166R1 Prohibiting basic_string And basic_string_view Construction From nullptr
 
 // Parallel Algorithms Notes
 // C++ allows an implementation to implement parallel algorithms as calls to the serial algorithms.

tests/std/tests/P0220R1_string_view/test.cpp

@@ -8,6 +8,7 @@
 #include <stdlib.h>
 #include <string>
 #include <string_view>
+#include <type_traits>
 
 #include <constexpr_char_traits.hpp>
 
@@ -1199,6 +1200,13 @@ static_assert(u8"abc"sv[1] == u8'b');
 static_assert(noexcept(u8"abc"sv));
 #endif // __cpp_char8_t
 
+// P2166R1 Prohibiting basic_string And basic_string_view Construction From nullptr
+#if _HAS_CXX23
+static_assert(!is_constructible_v<string_view, nullptr_t>, "constructing string_view from nullptr_t is prohibited");
+static_assert(!is_constructible_v<string, nullptr_t>, "constructing string from nullptr_t is prohibited");
+static_assert(!is_assignable_v<string&, nullptr_t>, "assigning nullptr_t to string is prohibited");
+#endif // _HAS_CXX23
+
 int main() {
     test_case_default_constructor();
     test_case_ntcts_constructor();