libstdc++: Make std::hash<basic_string<>> allocator-agnostic (LWG 3705)

This new library issue was recently moved to Tentatively Ready by an LWG
poll, so I'm making the change on trunk.

As noted in PR libstc++/105907 the std::hash specializations for PMR
strings were not treated as slow hashes by the unordered containers, so
this change preserves that. The new specializations for custom
allocators are also not treated as slow, for the same reason. For the
versioned namespace (i.e. unstable ABI) we don't have to worry about
that, so can enable hash code caching for all basic_string
specializations.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (__hash_str_base): New class
	template.
	(hash<basic_string<C, char_traits<C>, A>>): Define partial
	specialization for each of the standard character types.
	(hash<string>, hash<wstring>, hash<u8string>, hash<u16string>)
	(hash<u32string>): Remove explicit specializations.
	* include/std/string (__hash_string_base): Remove class
	template.
	(hash<pmr::string>, hash<pmr::wstring>, hash<pmr::u8string>)
	(hash<pmr::u16string>, hash<pmr::u32string>): Remove explicit
	specializations.
	* testsuite/21_strings/basic_string/hash/hash.cc: Test with
	custom allocators.
	* testsuite/21_strings/basic_string/hash/hash_char8_t.cc:
	Likewise.

2 files changed, 28 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/hash/hash.cc b/libstdc++-v3/testsuite/21_strings/basic_string/hash/hash.cc
index 4c16f0e6f40..7f3809926e5 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/hash/hash.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/hash/hash.cc
@@ -20,6 +20,7 @@
 #include <string>
 #include <memory_resource>
 #include <testsuite_hooks.h>
+#include <testsuite_allocator.h>
 
 // C++17 24.3.5 [basic.string.hash]
 // If S is one of these string types, SV is the corresponding string view type,
@@ -54,9 +55,24 @@ test02()
 #endif
 }
 
+template<typename C>
+using String
+  = std::basic_string<C, std::char_traits<C>, __gnu_test::SimpleAllocator<C>>;
+
+void
+test03()
+{
+  // LWG 3705. Hashability shouldn't depend on basic_string's allocator
+  VERIFY( test(String<char>("a narrow string")) );
+  VERIFY( test(String<char16_t>(u"a utf-16 string")) );
+  VERIFY( test(String<char32_t>(U"a utf-32 string")) );
+  VERIFY( test(String<wchar_t>(L"a wide string")) );
+}
+
 int
 main()
 {
   test01();
   test02();
+  test03();
 }
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/hash/hash_char8_t.cc b/libstdc++-v3/testsuite/21_strings/basic_string/hash/hash_char8_t.cc
index 217fe15c7fe..0ff98ad1caa 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/hash/hash_char8_t.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/hash/hash_char8_t.cc
@@ -21,6 +21,7 @@
 #include <string>
 #include <memory_resource>
 #include <testsuite_hooks.h>
+#include <testsuite_allocator.h>
 
 // C++2a N4810 21.3.5 [basic.string.hash]
 // If S is one of these string types, SV is the corresponding string view type,
@@ -55,9 +56,20 @@ test02()
   VERIFY( hash<std::string>()(native) == hash<std::u8string>()(utf8) );
 }
 
+void
+test03()
+{
+  using Alloc = __gnu_test::SimpleAllocator<char8_t>;
+  using Stringu8 = std::basic_string<char8_t, std::char_traits<char8_t>, Alloc>;
+
+  // LWG 3705. Hashability shouldn't depend on basic_string's allocator
+  VERIFY( test(Stringu8(u8"a utf-8 string, with custom allocator")) );
+}
+
 int
 main()
 {
   test01();
   test02();
+  test03();
 }