[libc++] Enable hash only for the correct types

[philnik]

Summary

Also implement LWG3705.
Fixes https://github.com/llvm/llvm-project/issues/55823

libcxx/docs/Status/Cxx2bIssues.csv

 "`3672 <https://wg21.link/LWG3672>`__","``common_iterator::operator->()`` should return by value","July 2022","","","|ranges|"
 "`3683 <https://wg21.link/LWG3683>`__","``operator==`` for ``polymorphic_allocator`` cannot deduce template argument in common cases","July 2022","",""
 "`3687 <https://wg21.link/LWG3687>`__","``expected<cv void, E>`` move constructor should move","July 2022","",""
 "`3692 <https://wg21.link/LWG3692>`__","``zip_view::iterator``'s ``operator<=>`` is overconstrained","July 2022","","","|spaceship|"
 "`3701 <https://wg21.link/LWG3701>`__","Make ``formatter<remove_cvref_t<const charT[N]>, charT>`` requirement explicit","July 2022","","","|format|"
 "`3702 <https://wg21.link/LWG3702>`__","Should ``zip_transform_view::iterator`` remove ``operator<``","July 2022","",""
 "`3703 <https://wg21.link/LWG3703>`__","Missing requirements for ``expected<T, E>`` requires ``is_void<T>``","July 2022","",""
 "`3704 <https://wg21.link/LWG3704>`__","LWG 2059 added overloads that might be ill-formed for sets","July 2022","",""
-"`3705 <https://wg21.link/LWG3705>`__","Hashability shouldn't depend on basic_string's allocator","July 2022","",""
+"`3705 <https://wg21.link/LWG3705>`__","Hashability shouldn't depend on basic_string's allocator","July 2022","|Complete|","16.0"
 "`3707 <https://wg21.link/LWG3707>`__","chunk_view::outer-iterator::value_type::size should return unsigned type","July 2022","","","|ranges|"
 "`3708 <https://wg21.link/LWG3708>`__","``take_while_view::sentinel``'s conversion constructor should move","July 2022","","","|ranges|"
 "`3709 <https://wg21.link/LWG3709>`__","LWG-3703 was underly ambitious","July 2022","",""
 "`3710 <https://wg21.link/LWG3710>`__","The ``end`` of ``chunk_view`` for input ranges can be ``const``","July 2022","","","|ranges|"
 "`3711 <https://wg21.link/LWG3711>`__","Missing preconditions for slide_view constructor","July 2022","","","|ranges|"
 "`3712 <https://wg21.link/LWG3712>`__","``chunk_view`` and ``slide_view`` should not be ``default_initializable``","July 2022","","","|ranges|"
 "`3713 <https://wg21.link/LWG3713>`__","Sorted with respect to comparator (only)","July 2022","",""
 "`3715 <https://wg21.link/LWG3715>`__","``view_interface::empty`` is overconstrained","July 2022","",""
 "`3719 <https://wg21.link/LWG3719>`__","Directory iterators should be usable with default sentinel","July 2022","",""
 "`3721 <https://wg21.link/LWG3721>`__","Allow an ``arg-id`` with a value of zero for ``width`` in ``std-format-spec``","July 2022","","","|format|"
 "`3724 <https://wg21.link/LWG3724>`__","``decay-copy`` should be constrained","July 2022","|Complete|","14.0"
 "","","","",""
 "`3645 <https://wg21.link/LWG3645>`__","``resize_and_overwrite`` is overspecified to call its callback with lvalues","Not voted in","|Complete|","14.0",""
 "`3631 <https://wg21.link/LWG3631>`__","``basic_format_arg(T&&)`` should use ``remove_cvref_t<T>`` throughout","Not voted in","|Complete|","15.0",""
 "","","","",""

libcxx/include/string

 #endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
 
 template<class _CharT, class _Traits, class _Allocator>
 _LIBCPP_TEMPLATE_DATA_VIS
 const typename basic_string<_CharT, _Traits, _Allocator>::size_type
                basic_string<_CharT, _Traits, _Allocator>::npos;
 
 template <class _CharT, class _Allocator>
-struct _LIBCPP_TEMPLATE_VIS
-    hash<basic_string<_CharT, char_traits<_CharT>, _Allocator> >
-    : public __unary_function<basic_string<_CharT, char_traits<_CharT>, _Allocator>, size_t>
+struct __string_hash : public __unary_function<basic_string<_CharT, char_traits<_CharT>, _Allocator>, size_t>
 {
     size_t
     operator()(const basic_string<_CharT, char_traits<_CharT>, _Allocator>& __val) const _NOEXCEPT
     { return __do_string_hash(__val.data(), __val.data() + __val.size()); }
 };
 
+template <class _Allocator>
+struct hash<basic_string<char, char_traits<char>, _Allocator> > : __string_hash<char, _Allocator> {};
+
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+template <class _Allocator>
+struct hash<basic_string<char8_t, char_traits<char8_t>, _Allocator> > : __string_hash<char8_t, _Allocator> {};
+#endif
+
+template <class _Allocator>
+struct hash<basic_string<char16_t, char_traits<char16_t>, _Allocator> > : __string_hash<char16_t, _Allocator> {};
+
+template <class _Allocator>
+struct hash<basic_string<char32_t, char_traits<char32_t>, _Allocator> > : __string_hash<char32_t, _Allocator> {};
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+template <class _Allocator>
+struct hash<basic_string<wchar_t, char_traits<wchar_t>, _Allocator> > : __string_hash<wchar_t, _Allocator> {};
+#endif
+
 template<class _CharT, class _Traits, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI basic_ostream<_CharT, _Traits>&
 operator<<(basic_ostream<_CharT, _Traits>& __os,
            const basic_string<_CharT, _Traits, _Allocator>& __str);
 
 template<class _CharT, class _Traits, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI basic_istream<_CharT, _Traits>&
 operator>>(basic_istream<_CharT, _Traits>& __is,

libcxx/include/string_view

 
 template<class _CharT, class _Traits>
 _LIBCPP_HIDE_FROM_ABI basic_ostream<_CharT, _Traits>&
 operator<<(basic_ostream<_CharT, _Traits>& __os,
            basic_string_view<_CharT, _Traits> __str);
 
 // [string.view.hash]
 template<class _CharT>
-struct _LIBCPP_TEMPLATE_VIS hash<basic_string_view<_CharT, char_traits<_CharT> > >
-    : public __unary_function<basic_string_view<_CharT, char_traits<_CharT> >, size_t>
+struct __string_view_hash : public __unary_function<basic_string_view<_CharT, char_traits<_CharT> >, size_t>
 {
     _LIBCPP_INLINE_VISIBILITY
     size_t operator()(const basic_string_view<_CharT, char_traits<_CharT> > __val) const _NOEXCEPT {
         return __do_string_hash(__val.data(), __val.data() + __val.size());
     }
 };
 
+template <>
+struct hash<basic_string_view<char, char_traits<char> > > : __string_view_hash<char> {};
+
+#ifndef _LIBCPP_HAS_NO_CHAR8_T
+template <>
+struct hash<basic_string_view<char8_t, char_traits<char8_t> > > : __string_view_hash<char8_t> {};
+#endif
+
+template <>
+struct hash<basic_string_view<char16_t, char_traits<char16_t> > > : __string_view_hash<char16_t> {};
+
+template <>
+struct hash<basic_string_view<char32_t, char_traits<char32_t> > > : __string_view_hash<char32_t> {};
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+template <>
+struct hash<basic_string_view<wchar_t, char_traits<wchar_t> > > : __string_view_hash<wchar_t> {};
+#endif
+
 #if _LIBCPP_STD_VER > 11
 inline namespace literals
 {
   inline namespace string_view_literals
   {
     inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
     basic_string_view<char> operator "" sv(const char *__str, size_t __len) _NOEXCEPT
     {

libcxx/test/std/strings/basic.string.hash/enabled_hashes.pass.cpp

 
 // <string>
 
 // Test that <string> provides all of the arithmetic, enum, and pointer
 // hash specializations.
 
 #include <string>
 
+#include "constexpr_char_traits.h"
 #include "poisoned_hash_helper.h"
-
+#include "test_allocator.h"
 #include "test_macros.h"
 
+struct MyChar {
+  char c;
+};
+
 int main(int, char**) {
   test_library_hash_specializations_available();
   {
     test_hash_enabled_for_type<std::string>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
     test_hash_enabled_for_type<std::wstring>();
 #endif
-#if defined(__cpp_lib_char8_t) && __cpp_lib_char8_t >= 201811L
+#ifndef TEST_HAS_NO_CHAR8_T
     test_hash_enabled_for_type<std::u8string>();
 #endif
     test_hash_enabled_for_type<std::u16string>();
     test_hash_enabled_for_type<std::u32string>();
+    test_hash_enabled_for_type<std::basic_string<char, std::char_traits<char>, test_allocator<char>>>();
+    test_hash_disabled_for_type<std::basic_string<MyChar, std::char_traits<MyChar>, std::allocator<MyChar>>>();
+    test_hash_disabled_for_type<std::basic_string<char, constexpr_char_traits<char>, std::allocator<char>>>();
   }
 
   return 0;
 }

libcxx/test/std/strings/string.view/string.view.hash/enabled_hashes.pass.cpp

 
 // <string_view>
 
 // Test that <string_view> provides all of the arithmetic, enum, and pointer
 // hash specializations.
 
 #include <string_view>
 
+#include "constexpr_char_traits.h"
 #include "poisoned_hash_helper.h"
-
 #include "test_macros.h"
 
+struct MyChar {
+  char c;
+};
+
 int main(int, char**) {
   test_library_hash_specializations_available();
   {
     test_hash_enabled_for_type<std::string_view>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
     test_hash_enabled_for_type<std::wstring_view>();
 #endif
-#if defined(__cpp_lib_char8_t) && __cpp_lib_char8_t >= 201811L
+#ifndef TEST_HAS_NO_CHAR8_T
     test_hash_enabled_for_type<std::u8string_view>();
 #endif
     test_hash_enabled_for_type<std::u16string_view>();
     test_hash_enabled_for_type<std::u32string_view>();
+    test_hash_disabled_for_type<std::basic_string_view<MyChar, std::char_traits<MyChar>>>();
+    test_hash_disabled_for_type<std::basic_string_view<char, constexpr_char_traits<char>>>();
   }
 
   return 0;
 }