[libcxx-commits] [libcxx] [libc++] Avoid constructing additional objects when using map::at (PR #157866)

[Louis Dionne via libcxx-commits]

================
@@ -1085,6 +1094,30 @@ public:
   _LIBCPP_HIDE_FROM_ABI mapped_type& operator[](key_type&& __k);
 #  endif
 
+  template <class _Arg,
+            __enable_if_t<is_convertible<_Arg, value_type>::value &&
+                              __desugars_to_v<__less_tag, key_compare, key_type, key_type> &&
----------------
ldionne wrote:

We should find a way to not list comparators explicitly here. This basically encodes whether we're allowed to assume that a comparator marked as transparent (via `is_transparent`) behaves like we'd think. This should be its own trait, and it should also be the target of a LWG issue clarifying the properties of transparent comparators.

If we had such clarification, then we could drop that trait because we could assume that a comparator marked with `is_transparent` behaves sanely.

The `enable_if` would become this:

```
is_convertible<_Arg, value_type>::value && // for standards conformance
!__is_true_transparent_comparator_v<__make_transparent_t<key_compare> > && // there exists an equivalent "true" transparent comparator
__is_transparently_comparable_v<key_type, __remove_cvref_t<_Arg> > // skipping the conversion is fine
```

https://github.com/llvm/llvm-project/pull/157866