[libcxx-commits] [PATCH] D103056: [libcxx][ranges] Add `ranges::transform_view`.

[Tim Song via Phabricator via libcxx-commits]

tcanens added inline comments.


================
Comment at: libcxx/include/__ranges/transform_view.h:289
+  friend constexpr decltype(auto) iter_move(const __iterator& __i)
+    noexcept(noexcept(*__i))
+  {
----------------
ldionne wrote:
> As it stands, I believe the Spec has an issue. CC @tcanens . I think `iter_move(transform_view::iterator)` is never `noexcept`. It is defined in http://eel.is/c++draft/range.transform.iterator as:
> 
> ```
> constexpr decltype(auto) iter_move(const transform_view::iterator& i)
>     noexcept(noexcept(invoke(*i.parent_->fun_, *i.current_))) {
>     if constexpr (is_lvalue_reference_v<decltype(*i)>)
>         return std::move(*i);
>     else
>         return *i;
> }
> ```
> 
> However, `*i.parent_->fun_` dereferences the `copyable-box` holding the function object, but `copyable-box` is specified to have the same API as `std::optional`. In the spec, `std::optional::operator*` is not `noexcept`, so the whole `noexcept(noexcept(invoke(*i.parent_->fun_, *i.current_)))` is always `noexcept(false)` unless the library provides a `noexcept` `std::optional::operator*` as an extension.
> 
> @tcanens Does that make sense? Is this LWG issue worthy?
> 
> (marking as Done since it's not an issue with this review per se).
This will be fixed by https://cplusplus.github.io/LWG/issue2762


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  https://reviews.llvm.org/D103056/new/

https://reviews.llvm.org/D103056