[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/docs/Status/Cxx2bPapers.csv

@@ -21,5 +21,5 @@
 "`P1951R1 <https://wg21.link/P1951R1>`__","LWG","Default Arguments for pair Forwarding Constructor","June 2021","",""
 "`P1989R2 <https://wg21.link/P1989R2>`__","LWG","Range constructor for std::string_view","June 2021","",""
 "`P2136R3 <https://wg21.link/P2136R3>`__","LWG","invoke_r","June 2021","",""
-"`P2166R1 <https://wg21.link/P2166R1>`__","LWG","A Proposal to Prohibit std::basic_string and std::basic_string_view construction from nullptr","June 2021","",""
+"`P2166R1 <https://wg21.link/P2166R1>`__","LWG","A Proposal to Prohibit std::basic_string and std::basic_string_view construction from nullptr","June 2021","|Complete|","13.0"
 "","","","","",""

libcxx/include/string

@@ -110,6 +110,7 @@ public:
         explicit basic_string(const T& t, const Allocator& a = Allocator()); // C++17
     basic_string(const value_type* s, const allocator_type& a = allocator_type());
     basic_string(const value_type* s, size_type n, const allocator_type& a = allocator_type());
+    basic_string(nullptr_t) = delete; // C++2b
     basic_string(size_type n, value_type c, const allocator_type& a = allocator_type());
     template<class InputIterator>
         basic_string(InputIterator begin, InputIterator end,
@@ -130,6 +131,7 @@ public:
              allocator_type::propagate_on_container_move_assignment::value ||
              allocator_type::is_always_equal::value ); // C++17
     basic_string& operator=(const value_type* s);
+    basic_string& operator=(nullptr_t) = delete; // C++2b
     basic_string& operator=(value_type c);
     basic_string& operator=(initializer_list<value_type>);
 
@@ -843,6 +845,10 @@ public:
         _LIBCPP_INLINE_VISIBILITY
         basic_string(const _CharT* __s, const _Allocator& __a);
 
+#if _LIBCPP_STD_VER > 20
+    basic_string(nullptr_t) = delete;
+#endif
+
     _LIBCPP_INLINE_VISIBILITY
     basic_string(const _CharT* __s, size_type __n);
     _LIBCPP_INLINE_VISIBILITY
@@ -906,6 +912,9 @@ public:
     basic_string& operator=(initializer_list<value_type> __il) {return assign(__il.begin(), __il.size());}
 #endif
     _LIBCPP_INLINE_VISIBILITY basic_string& operator=(const value_type* __s) {return assign(__s);}
+#if _LIBCPP_STD_VER > 20
+    basic_string& operator=(nullptr_t) = delete;
+#endif
     basic_string& operator=(value_type __c);
 
 #if _LIBCPP_DEBUG_LEVEL == 2

libcxx/include/string_view

@@ -83,6 +83,7 @@ namespace std {
       basic_string_view& operator=(const basic_string_view&) noexcept = default;
       template<class Allocator>
       constexpr basic_string_view(const charT* str);
+      basic_string_view(nullptr_t) = delete; // C++2b
       constexpr basic_string_view(const charT* str, size_type len);
 
       // 7.4, basic_string_view iterator support
@@ -273,6 +274,10 @@ public:
     basic_string_view(const _CharT* __s)
         : __data(__s), __size(_VSTD::__char_traits_length_checked<_Traits>(__s)) {}
 
+#if _LIBCPP_STD_VER > 20
+    basic_string_view(nullptr_t) = delete;
+#endif
+
     // [string.view.iterators], iterators
     _LIBCPP_CONSTEXPR _LIBCPP_INLINE_VISIBILITY
     const_iterator begin()  const _NOEXCEPT { return cbegin(); }

libcxx/test/libcxx/debug/db_string_view.pass.cpp

@@ -21,18 +21,22 @@
 #include "debug_mode_helper.h"
 
 void test_null_argument() {
-  EXPECT_DEATH(std::string_view(nullptr));
-  EXPECT_DEATH(std::string_view(NULL));
+  // C++2b prohibits construction of string_view from nullptr_t.
+  const char* nullp = nullptr;
+  const char* null = NULL;
+  (void)nullp;
+  (void)null;
+  EXPECT_DEATH((std::string_view(nullp)));
+  EXPECT_DEATH((std::string_view(null)));
   EXPECT_DEATH(std::string_view(static_cast<const char*>(0)));
   {
     std::string_view v;
-    EXPECT_DEATH(((void)(v == nullptr)));
-    EXPECT_DEATH(((void)(nullptr == v)));
+    EXPECT_DEATH(((void)(v == nullp)));
+    EXPECT_DEATH(((void)(nullp == v)));
   }
 }
 
-int main(int, char**)
-{
+int main(int, char**) {
   test_null_argument();
 
   return 0;

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

@@ -0,0 +1,21 @@
+//===----------------------------------------------------------------------===//
+//
+// 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>
+
+// basic_string(nullptr_t) = delete; // C++2b
+// basic_string& operator=(nullptr_t) = delete; // C++2b
+
+#include <string>
+#include <type_traits>
+
+static_assert(!std::is_convertible_v<decltype(nullptr), std::string>);
+static_assert(!std::is_constructible_v<std::string, decltype(nullptr)>);
+static_assert(!std::is_assignable_v<std::string, decltype(nullptr)>);

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)>);