[libc++][modules] Refactor poisoned_hash_helper (#108296)

The poisoned_hash_helper header was relying on an implicit forward
declaration of std::hash located in <type_traits>. When we improve the
modularization of the library, that causes issues, in addition to being
a fundamentally non-portable assumption in the test suite.

It turns out that the reason for relying on a forward declaration is to
be able to test that std::hash is *not* provided if we don't include any
header that provides it. But testing that is actually both non-portable
and not really useful.

Indeed, what harm does it make if additional headers provide std::hash
specializations? That would certainly be conforming -- the Standard
never requires an implementation to avoid providing a declaration when a
given header is included, instead it mandates what *must* be provided
for sure. In that spirit, it would be conforming for e.g. `<cstddef>` to
define the hash specializations if that was our desire. I also don't
read https://wg21.link/P0513R0 as going against that statement. Hence,
this patch just removes that test which doesn't carry its weight.

Fixes #56938

1 files changed, 7 insertions, 7 deletions

diff --git a/libcxx/test/std/strings/string.view/string.view.hash/enabled_hashes.pass.cpp b/libcxx/test/std/strings/string.view/string.view.hash/enabled_hashes.pass.cpp
index b2ffd2010838..13abb9456936 100644
--- a/libcxx/test/std/strings/string.view/string.view.hash/enabled_hashes.pass.cpp
+++ b/libcxx/test/std/strings/string.view/string.view.hash/enabled_hashes.pass.cpp
@@ -53,17 +53,17 @@ struct std::char_traits<MyChar> {
 int main(int, char**) {
   test_library_hash_specializations_available();
   {
-    test_hash_enabled_for_type<std::string_view>();
+    test_hash_enabled<std::string_view>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
-    test_hash_enabled_for_type<std::wstring_view>();
+    test_hash_enabled<std::wstring_view>();
 #endif
 #ifndef TEST_HAS_NO_CHAR8_T
-    test_hash_enabled_for_type<std::u8string_view>();
+    test_hash_enabled<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>>>();
+    test_hash_enabled<std::u16string_view>();
+    test_hash_enabled<std::u32string_view>();
+    test_hash_disabled<std::basic_string_view<MyChar, std::char_traits<MyChar>>>();
+    test_hash_disabled<std::basic_string_view<char, constexpr_char_traits<char>>>();
   }
 
   return 0;