[libcxx-commits] [PATCH] D99873: [libcxx] adds `std::ranges::iter_move` and `std::iter_rvalue_reference_t

[Christopher Di Bella via Phabricator via libcxx-commits]

cjdb added inline comments.


================
Comment at: libcxx/include/iterator:2475
+  // ```
+  template<__unqualified_iter_move _Ip>
+  constexpr decltype(auto) operator()(_Ip&& __i) const
----------------
miscco wrote:
> I am slightly concerned about the number of overloads that are introduced and the instantiation cost, which is quite high on clang right now due to lack of memoization of concepts.
> 
> I would rather put everything in a single `operator()` and then use `if constexpr` to return the right thing.
> 
> That would enable us to simplify the concept definitions, as we do not need to exclude the previous concepts  
> which is quite high on clang right now due to lack of memoization of concepts.

How long ago did Clang disable memoisation?

> I would rather put everything in a single `operator()` and then use `if constexpr` to return the right thing.

I'm aware of this optimisation from GCC Concepts TS days, but I'd like to see evidence that Clang will get a significant performance boost from this too.
It'll also mean that we can't have conditional noexcept, something I'm reluctant to give up.


Repository:
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  https://reviews.llvm.org/D99873/new/

https://reviews.llvm.org/D99873