[libc++] Specialize __lazy_synth_three_way_comparator for std::greater and friends

[philnik777]

This specializes __lazy_synth_three_way_comparator to forward to __default_three_way_comparator if the comparator desugars to __greater_tag. This is the same as the desugaring to __less_tag except that the sign has to be inverted.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

This specializes __lazy_synth_three_way_comparator to forward to __default_three_way_comparator if the comparator desugars to __greater_tag. This is the same as the desugaring to __less_tag except that the sign has to be inverted.

---

Full diff: https://github.com/llvm/llvm-project/pull/157624.diff

1 Files Affected:

• (modified) libcxx/include/__utility/lazy_synth_three_way_comparator.h (+17)

diff --git a/libcxx/include/__utility/lazy_synth_three_way_comparator.h b/libcxx/include/__utility/lazy_synth_three_way_comparator.h
index ca98845f04191..635dee1f7e4cb 100644
--- a/libcxx/include/__utility/lazy_synth_three_way_comparator.h
+++ b/libcxx/include/__utility/lazy_synth_three_way_comparator.h
@@ -85,6 +85,23 @@ struct __lazy_synth_three_way_comparator<_Comparator,
   }
 };
 
+template <class _Comparator, class _LHS, class _RHS>
+struct __lazy_synth_three_way_comparator<_Comparator,
+                                         _LHS,
+                                         _RHS,
+                                         __enable_if_t<__desugars_to_v<__greater_tag, _Comparator, _LHS, _RHS> &&
+                                                       __has_default_three_way_comparator_v<_LHS, _RHS> > > {
+  // This lifetimebound annotation is technically incorrect, but other specializations actually capture the lifetime of
+  // the comparator.
+  _LIBCPP_HIDE_FROM_ABI __lazy_synth_three_way_comparator(_LIBCPP_CTOR_LIFETIMEBOUND const _Comparator&) {}
+
+  // Same comment as above.
+  _LIBCPP_HIDE_FROM_ABI static __eager_compare_result
+  operator()(_LIBCPP_LIFETIMEBOUND const _LHS& __lhs, _LIBCPP_LIFETIMEBOUND const _RHS& __rhs) {
+    return __eager_compare_result(-__default_three_way_comparator<_LHS, _RHS>()(__lhs, __rhs));
+  }
+};
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___UTILITY_LAZY_SYNTH_THREE_WAY_COMPARATOR_H

[ldionne]

LGTM with added tests as suggested.

[github-actions]

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