[libc++] Implement P2499R0 (`string_view` range constructor should be `explicit`)

[fsb4000]

fix diff

[philnik]

LGTM % nits. Do you need someone to land it? If yes, please provide "Your Name" <[e-mail]>.

libcxx/docs/ReleaseNotes.rst

40

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

189

I wouldn't bother saying which paper changed it. It's not like anybody will care when reading this later.

[fsb4000]

Yes.

"Igor Zhukov" <[e-mail]>

I will fix the tests.

[fsb4000]

fix tests

[fsb4000]

remove mention of paper in tests

[fsb4000]

remove "Implemented" in ReleaseNotes.rst

[jloser]

LGTM!

[fsb4000]

fix std\strings\string.view\string.view.deduct\range.pass.cpp

[Mordante]

LGTM modulo one nit.

libcxx/docs/ReleaseNotes.rst

40

This is the style we switched to in the previous release, so let's keep using that.

[fsb4000]

new style of implemented papers in ReleaseNotes.rst

[ldionne]

This was accepted as a paper, not as a LWG issue/DR, right? If so, why is this not guarded with a version check for the C+ Standard?

[fsb4000]

Hi Louis Dionne!

This is tested above: https://github.com/llvm/llvm-project/blob/8be1197285bdff6e03bf26b77052daa710f73068/libcxx/include/string_view#L319

#if _LIBCPP_STD_VER > 20 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)

So this is C++23 change.

[philnik]

In D130785#3693686, @ldionne wrote:

This was accepted as a paper, not as a LWG issue/DR, right? If so, why is this not guarded with a version check for the C+ Standard?

Both the addition of the constructor itself and this change are part of C++23.

[ldionne]

Sorry, I looked way too quickly at this patch. Thanks for the patch and for correcting me!

libcxx/docs/ReleaseNotes.rst

 the current one. To see the release notes for a specific release, please
 see the `releases page <https://llvm.org/releases/>`_.
 
 What's New in Libc++ 16.0.0?
 ============================
 
 Implemented Papers
 ------------------
+- Implemented P2499R0 (``string_view`` range constructor should be ``explicit``)
 
 Improvements and New Features
 -----------------------------
 
 Deprecations and Removals
 -------------------------
 
 Upcoming Deprecations and Removals
 ----------------------------------
 
 ABI Affecting Changes
 ---------------------
 
 Build System Changes
 --------------------

libcxx/docs/Status/Cxx2bPapers.csv

 "`P2438R2 <https://wg21.link/P2438R2>`__","LWG","``std::string::substr() &&``","July 2022","",""
 "`P2445R1 <https://wg21.link/P2445R1>`__","LWG","``forward_like``","July 2022","",""
 "`P2446R2 <https://wg21.link/P2446R2>`__","LWG","``views::as_rvalue``","July 2022","",""
 "`P2460R2 <https://wg21.link/P2460R2>`__","LWG","Relax requirements on ``wchar_t`` to match existing practices","July 2022","",""
 "`P2465R3 <https://wg21.link/P2465R3>`__","LWG","Standard Library Modules ``std`` and ``std.compat``","July 2022","",""
 "`P2467R1 <https://wg21.link/P2467R1>`__","LWG","Support exclusive mode for ``fstreams``","July 2022","",""
 "`P2474R2 <https://wg21.link/P2474R2>`__","LWG","``views::repeat``","July 2022","",""
 "`P2494R2 <https://wg21.link/P2494R2>`__","LWG","Relaxing range adaptors to allow for move only types","July 2022","",""
-"`P2499R0 <https://wg21.link/P2499R0>`__","LWG","``string_view`` range constructor should be ``explicit``","July 2022","",""
+"`P2499R0 <https://wg21.link/P2499R0>`__","LWG","``string_view`` range constructor should be ``explicit``","July 2022","|Complete|","16.0"
 "`P2502R2 <https://wg21.link/P2502R2>`__","LWG","``std::generator``: Synchronous Coroutine Generator for Ranges","July 2022","",""
 "`P2508R1 <https://wg21.link/P2508R1>`__","LWG","Exposing ``std::basic-format-string``","July 2022","",""
 "`P2513R4 <https://wg21.link/P2513R4>`__","LWG","``char8_t`` Compatibility and Portability Fixes","July 2022","",""
 "`P2517R1 <https://wg21.link/P2517R1>`__","LWG","Add a conditional ``noexcept`` specification to ``std::apply``","July 2022","",""
 "`P2520R0 <https://wg21.link/P2520R0>`__","LWG","``move_iterator`` should be a random access iterator","July 2022","",""
 "`P2540R1 <https://wg21.link/P2540R1>`__","LWG","Empty Product for certain Views","July 2022","",""
 "`P2549R1 <https://wg21.link/P2549R1>`__","LWG","``std::unexpected`` should have ``error()`` as member accessor","July 2022","",""
 "`P2553R1 <https://wg21.link/P2553R1>`__","LWG","Make ``mdspan`` ``size_type`` controllable","July 2022","",""
 "`P2554R0 <https://wg21.link/P2554R0>`__","LWG","C-Array Interoperability of MDSpan","July 2022","",""
 "`P2585R0 <https://wg21.link/P2585R0>`__","LWG","Improving default container formatting","July 2022","",""
 "`P2590R2 <https://wg21.link/P2590R2>`__","LWG","Explicit lifetime management","July 2022","",""
 "`P2599R2 <https://wg21.link/P2599R2>`__","LWG","``mdspan::size_type`` should be ``index_type``","July 2022","",""
 "`P2604R0 <https://wg21.link/P2604R0>`__","LWG","mdspan: rename pointer and contiguous","July 2022","",""
 "`P2613R1 <https://wg21.link/P2613R1>`__","LWG","Add the missing ``empty`` to ``mdspan``","July 2022","",""
 "","","","","",""

libcxx/include/string_view

template <class _Range>
         !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 _LIBCPP_HIDE_FROM_ABI
+    constexpr explicit _LIBCPP_HIDE_FROM_ABI
     basic_string_view(_Range&& __r) : __data(ranges::data(__r)), __size(ranges::size(__r)) {}
 #endif // _LIBCPP_STD_VER > 20 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
 
     _LIBCPP_CONSTEXPR _LIBCPP_INLINE_VISIBILITY
     basic_string_view(const _CharT* __s)
         : __data(__s), __size(_VSTD::__char_traits_length_checked<_Traits>(__s)) {}
 
 #if _LIBCPP_STD_VER > 20

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

#endif
   static_assert(std::is_constructible_v<std::string_view, DeletedConversionOperator>);
   static_assert(std::is_constructible_v<std::string_view, const DeletedConversionOperator>);
   static_assert(std::is_constructible_v<std::string_view, DeletedConstConversionOperator>);
   static_assert(std::is_constructible_v<std::string_view, const DeletedConstConversionOperator>);
 
   // Test that we're not trying to use the type's conversion operator to string_view in the constructor.
   {
     const DeletedConversionOperator d;
-    std::basic_string_view<char> csv = d;
+    std::basic_string_view<char> csv = std::basic_string_view<char>(d);
     assert(csv == "test");
   }
 
   {
     DeletedConstConversionOperator dc;
-    std::basic_string_view<char> sv = dc;
+    std::basic_string_view<char> sv = std::basic_string_view<char>(dc);
     assert(sv == "test");
   }
 
   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>&>);
try {
     (void) std::string_view(x);
     assert(false);
   } catch (int i) {
     assert(i == 42);
   }
 }
 #endif
 
+static_assert(!std::is_convertible_v<std::vector<char>, std::string_view>);
+
 int main(int, char**) {
   test();
   static_assert(test());
 #ifndef TEST_HAS_NO_EXCEPTIONS
   test_throwing();
 #endif
 
   return 0;
 }