[libc++] LWG 3857: allow `string_view` conversion when only traits vary

[jloser]

Summary

The basic_string_view constructor accepting a contiguous range rejects
converting between basic_string_view even when only the trait types vary.
This prevents conversions for converting from basic_string_view<C, T1> and
basic_string<C, T1, A> to basic_string_view<C, T2>. Recently, this
constructor was made explicit, so there's no reason to really forbid this
conversion anymore.

Relax the restriction that the trait types need to match in this constructor.

[jloser]

Undo extra clang-format in from_range.pass.cpp

[jloser]

libcxx/test/std/strings/string.view/string.view.cons/from_range.pass.cpp

145–146

I slightly prefer ripping this (and the few lines above it) out entirely. It was originally just to model this constraint failure case. How do others feel? I've left it in for now, but it's not testing anything too useful now IMO.

[philnik]

LGTM.

libcxx/test/std/strings/string.view/string.view.cons/from_range.pass.cpp

145–146

I think we can remove the static_asserts, since we have a runtime test now.

[jloser]

Drop the static_assert tests that verified traits_type mismatches as they are no longer relevant

libcxx/test/std/strings/string.view/string.view.cons/from_range.pass.cpp

145–146

SGTM — just removed them.

[jloser]

@Mordante do you want me to wait until you land https://reviews.llvm.org/D143845? Or, you can just feel free to mark LWG 3857 as Completed in LLVM 17.0.0 if I land this first — whatever you prefer.

[jloser]

Remove unnecessary constexpr_char_traits.h include

[jloser]

Rebase since the clang-format requirement patch landed

[Mordante]

In D143972#4126093, @jloser wrote:

@Mordante do you want me to wait until you land https://reviews.llvm.org/D143845? Or, you can just feel free to mark LWG 3857 as Completed in LLVM 17.0.0 if I land this first — whatever you prefer.

I've landed my change, can you rebased and update the issue list before landing.

[ldionne]

Awesome, thanks for being on top of this! LGTM w/ rebase on top of LWG issues list.

[jloser]

Rebase and mark the paper status as completed

libcxx/docs/Status/Cxx2bIssues.csv

 "`3839 <https://wg21.link/LWG3839>`__","``range_formatter``'s ``set_separator``, ``set_brackets``, and ``underlying`` functions should be ``noexcept``","February 2023","","","|format|"
 "`3841 <https://wg21.link/LWG3841>`__","``<version>`` should not be ""all freestanding""","February 2023","","",""
 "`3842 <https://wg21.link/LWG3842>`__","Unclear wording for ``precision`` in ``chrono-format-spec``","February 2023","","","|format|"
 "`3848 <https://wg21.link/LWG3848>`__","``adjacent_view``, ``adjacent_transform_view`` and ``slide_view`` missing ``base`` accessor","February 2023","","","|ranges|"
 "`3849 <https://wg21.link/LWG3849>`__","``cartesian_product_view::iterator``'s default constructor is overconstrained","February 2023","","","|ranges|"
 "`3850 <https://wg21.link/LWG3850>`__","``views::as_const`` on ``empty_view<T>`` should return ``empty_view<const T>``","February 2023","","","|ranges|"
 "`3851 <https://wg21.link/LWG3851>`__","``chunk_view::inner-iterator`` missing custom ``iter_move`` and ``iter_swap``","February 2023","","","|ranges|"
 "`3853 <https://wg21.link/LWG3853>`__","``basic_const_iterator<volatile int*>::operator->`` is ill-formed","February 2023","","",""
-"`3857 <https://wg21.link/LWG3857>`__","``basic_string_view`` should allow explicit conversion when only traits vary","February 2023","","",""
+"`3857 <https://wg21.link/LWG3857>`__","``basic_string_view`` should allow explicit conversion when only traits vary","February 2023","|Complete|","17.0",""
 "`3860 <https://wg21.link/LWG3860>`__","``range_common_reference_t`` is missing","February 2023","","","|ranges|"
 "`3866 <https://wg21.link/LWG3866>`__","Bad Mandates for ``expected::transform_error`` overloads","February 2023","","",""
 "`3867 <https://wg21.link/LWG3867>`__","Should ``std::basic_osyncstream``'s move assignment operator be ``noexcept``?","February 2023","","",""
 "`3441 <https://wg21.link/LWG3441>`__","Misleading note about calls to customization points","February 2023","","",""
 "`3622 <https://wg21.link/LWG3622>`__","Misspecified transitivity of equivalence in §[unord.req.general]","February 2023","","",""
 "`3631 <https://wg21.link/LWG3631>`__","``basic_format_arg(T&&)`` should use ``remove_cvref_t<T>`` throughout","February 2023","|Complete|","15.0",""
 "`3645 <https://wg21.link/LWG3645>`__","``resize_and_overwrite`` is overspecified to call its callback with lvalues","February 2023","|Complete|","14.0",""
 "`3655 <https://wg21.link/LWG3655>`__","The ``INVOKE`` operation and union types","February 2023","","",""

libcxx/include/string_view

template <class _Range>
       requires (
         !is_same_v<remove_cvref_t<_Range>, basic_string_view> &&
         ranges::contiguous_range<_Range> &&
         ranges::sized_range<_Range> &&
         is_same_v<ranges::range_value_t<_Range>, _CharT> &&
         !is_convertible_v<_Range, const _CharT*> &&
         (!requires(remove_cvref_t<_Range>& __d) {
           __d.operator _VSTD::basic_string_view<_CharT, _Traits>();
-        }) &&
-        (!requires {
-         typename remove_reference_t<_Range>::traits_type;
-        } || is_same_v<typename remove_reference_t<_Range>::traits_type, _Traits>)
+        })
       )
     constexpr explicit _LIBCPP_HIDE_FROM_ABI
     basic_string_view(_Range&& __r) : __data_(ranges::data(__r)), __size_(ranges::size(__r)) {}
 #endif // _LIBCPP_STD_VER >= 23
 
     _LIBCPP_CONSTEXPR _LIBCPP_INLINE_VISIBILITY
     basic_string_view(const _CharT* __s)
         : __data_(__s), __size_(_VSTD::__char_traits_length_checked<_Traits>(__s)) {}

libcxx/test/std/strings/string.view/string.view.cons/from_range.pass.cpp

 #include <string_view>
 #include <array>
 #include <cassert>
 #include <iterator>
 #include <ranges>
 #include <type_traits>
 #include <vector>
 
-#include "constexpr_char_traits.h"
 #include "make_string.h"
 #include "test_iterators.h"
 #include "test_range.h"
 
 template<class CharT>
 constexpr void test() {
   auto data = MAKE_STRING_VIEW(CharT, "test");
   std::array<CharT, 4> arr;
#endif
   }
 
   {
     DeletedConstConversionOperator dc;
     std::basic_string_view<char> sv = std::basic_string_view<char>(dc);
     assert(sv == "test");
   }
 
+  // Different trait types
+  {
+    struct OtherTraits : std::char_traits<char> {};
+    std::basic_string_view<char> sv1{"hello"};
+    std::basic_string_view<char, OtherTraits> sv2(sv1);
+    assert(sv1.size() == sv2.size());
+    assert(sv1.data() == sv2.data());
+  }
+
   return true;
 }
 
 static_assert(std::is_constructible_v<std::string_view, std::vector<char>&>);
 static_assert(std::is_constructible_v<std::string_view, const std::vector<char>&>);
 static_assert(std::is_constructible_v<std::string_view, std::vector<char>&&>);
 static_assert(std::is_constructible_v<std::string_view, const std::vector<char>&&>);
 

 
 struct WithStringViewConversionOperator {
   char* begin() const;
   char* end() const;
   operator std::string_view() const { return {}; }
 };
 
 static_assert(std::is_constructible_v<std::string_view, WithStringViewConversionOperator>); // lvalue
 static_assert(std::is_constructible_v<std::string_view, const WithStringViewConversionOperator&>); // const lvalue
-static_assert(std::is_constructible_v<std::string_view, WithStringViewConversionOperator&&>); // rvalue
+static_assert(std::is_constructible_v<std::string_view, WithStringViewConversionOperator&&>);      // rvalue
 
-template <class CharTraits>
-struct WithTraitsType {
-  typename CharTraits::char_type* begin() const;
-  typename CharTraits::char_type* end() const;
-  using traits_type = CharTraits;
-};
-
-using CCT = constexpr_char_traits<char>;
-static_assert(std::is_constructible_v<std::string_view, WithTraitsType<std::char_traits<char>>>);
-#ifndef TEST_HAS_NO_WIDE_CHARACTERS
-static_assert(std::is_constructible_v<std::wstring_view, WithTraitsType<std::char_traits<wchar_t>>>);
-#endif
-static_assert(std::is_constructible_v<std::basic_string_view<char, CCT>, WithTraitsType<CCT>>);
-static_assert(!std::is_constructible_v<std::string_view, WithTraitsType<CCT>>);  // wrong traits type
-#ifndef TEST_HAS_NO_WIDE_CHARACTERS
-static_assert(!std::is_constructible_v<std::wstring_view, WithTraitsType<std::char_traits<char>>>);  // wrong traits type
-#endif
-
 #ifndef TEST_HAS_NO_EXCEPTIONS
 void test_throwing() {
   struct ThrowingData {
     char* begin() const { return nullptr; }
     char* end() const { return nullptr; }
     char* data() const { throw 42; return nullptr; }
   };
   try {

libcxx/test/std/strings/string.view/string.view.cons/from_string1.compile.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-// 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: !stdlib=libc++ && (c++03 || c++11 || c++14)
-
-// <string_view>
-
-// template<class Allocator>
-// basic_string_view(const basic_string<_CharT, _Traits, Allocator>& _str) noexcept
-
-#include <string_view>
-#include <string>
-#include <cassert>
-
-struct dummy_char_traits : public std::char_traits<char> {};
-
-int main(int, char**) {
-    using string_view = std::basic_string_view<char>;
-    using string      = std::              basic_string     <char, dummy_char_traits>;
-
-    {
-    string s{"QBCDE"};
-    string_view sv1 ( s );
-    assert ( sv1.size() == s.size());
-    assert ( sv1.data() == s.data());
-    }
-
-  return 0;
-}

libcxx/test/std/strings/string.view/string.view.cons/from_string2.compile.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-// 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: !stdlib=libc++ && (c++03 || c++11 || c++14)
-
-// <string_view>
-
-// template<class Allocator>
-// basic_string_view(const basic_string<_CharT, _Traits, Allocator>& _str) noexcept
-
-#include <string_view>
-#include <string>
-#include <cassert>
-
-struct dummy_char_traits : public std::char_traits<char> {};
-
-int main(int, char**) {
-    using string_view = std::basic_string_view<char, dummy_char_traits>;
-    using string      = std::              basic_string     <char>;
-
-    {
-    string s{"QBCDE"};
-    string_view sv1 ( s );
-    assert ( sv1.size() == s.size());
-    assert ( sv1.data() == s.data());
-    }
-
-  return 0;
-}