[compiler-rt] [libcxx] Unifying __is_trivial_equality_predicate and __is_trivial_plus_operation into __desugars_to (PR #68642)

[Louis Dionne via llvm-commits]

================
@@ -29,12 +30,11 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <
-    typename _DifferenceType,
-    typename _Tp,
-    typename _BinaryOperation,
-    typename _UnaryOperation,
-    __enable_if_t<__is_trivial_plus_operation<_BinaryOperation, _Tp, _Tp>::value && is_arithmetic_v<_Tp>, int> = 0>
+template < typename _DifferenceType,
+           typename _Tp,
+           typename _BinaryOperation,
+           typename _UnaryOperation,
+           __enable_if_t<__desugars_to<_BinaryOperation, plus<_Tp> >::value && is_arithmetic_v<_Tp>, int> = 0>
----------------
ldionne wrote:

With the changes below, this comment would become:

```
template < typename _DifferenceType,
            typename _Tp,
            typename _BinaryOperation,
            typename _UnaryOperation,
            typename _UnaryResult = invoke_result_t<_UnaryOperation, _DifferenceType>,
            __enable_if_t<__desugars_to<__plus_tag, _BinaryOperation, _Tp, _UnaryResult>::value && is_arithmetic_v<_Tp>, int> = 0>
```

Also worth thinking whether we really need/want to restrict this to arithmetic types, since I could definitely imagine doing a reduction over e.g. strings. String concatenation is not commutative, but it is associative. Should this work?

https://github.com/llvm/llvm-project/pull/68642