[libcxx-commits] [PATCH] D99873: [libcxx] adds `std::ranges::iter_move` and `std::iter_rvalue_reference_t
Michael Schellenberger Costa via Phabricator via libcxx-commits
libcxx-commits at lists.llvm.org
Wed Apr 7 23:08:49 PDT 2021
miscco added inline comments.
================
Comment at: libcxx/include/iterator:2498
+
+ // Otherwise, `ranges::iter_move(E)` is ill-formed.
+ void operator()(auto&&) const = delete;
----------------
What is it with those strange `!`
================
Comment at: libcxx/include/iterator:2475
+ // ```
+ template<__unqualified_iter_move _Ip>
+ constexpr decltype(auto) operator()(_Ip&& __i) const
----------------
tcanens wrote:
> cjdb wrote:
> > 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.
> Conditional noexcept is required for conformance since the wording uses expression-equivalent.
Note that we do have conditional noexcept with MSVC and I believe libstdc++
You have have a look at the `_Choice` machinery @CaseyCarter build. It is super helpful
Repository:
rG LLVM Github Monorepo
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https://reviews.llvm.org/D99873/new/
https://reviews.llvm.org/D99873
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