[libc++] [c++2b] [P2166] Prohibit string and string_view construction from nullptr.

* https://wg21.link/P2166

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D106801

libcxx/test/std/strings/string.view/string.view.cons/nullptr.compile.pass.cpp

@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <string_view>
+
+// basic_string_view(nullptr_t) = delete; // C++2b
+
+#include <string_view>
+#include <type_traits>
+
+static_assert(!std::is_convertible_v<decltype(nullptr), std::string_view>);
+static_assert(!std::is_constructible_v<std::string_view, decltype(nullptr)>);
+static_assert(!std::is_assignable_v<std::string_view, decltype(nullptr)>);