Different deduction guides in paper and implementation of `indirect`

[Ukilele]

Hey, I just realized that the deduction guides described in the proposal D3019R12 are different from the ones in the implementation of indirect.

Proposal:

template <class Value>
indirect(Value) -> indirect<Value>;

template <class Allocator, class Value>
indirect(allocator_arg_t, Allocator, Value) -> indirect<Value,
         typename allocator_traits<Allocator>::template rebind_alloc<Value>>;

Implementation

template <typename Value>
indirect(Value) -> indirect<Value>;

template <typename Value>
indirect(std::in_place_t, Value) -> indirect<Value>;

template <typename Alloc, typename Value>
indirect(std::allocator_arg_t, Alloc, std::in_place_t, Value) -> indirect<
    Value, typename std::allocator_traits<Alloc>::template rebind_alloc<Value>>;

Not sure what the right solution here is, but I at least wanted to report it asap.

[Ukilele]

While we are at it, a small follow-up question regarding deduction guides and CTAD: Does https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1518r2.html somehow also apply to indirect?

I.e. should it be:

- constexpr indirect(allocator_arg_t, const Allocator& a, const indirect& other);
+ constexpr indirect(allocator_arg_t, const type_identity_t<Allocator>& a, const indirect& other);

- constexpr indirect(allocator_arg_t, const Allocator& a, indirect&& other);
+ constexpr indirect(allocator_arg_t, const type_identity_t<Allocator>& a, indirect&& other);

I am not sure, but I got reminded of P1518 and at least wanted to mention it.

[jbcoe]

Thanks @Ukilele. The implementation should be brought in line with the proposal (on this front and others).

[jbcoe]

I'm not sure about P1518. I'll have to dig in and follow up next week. What do you think the right direction is?

Possibly related: microsoft/STL#4060

We're currently consistent with tuple: https://eel.is/c++draft/tuple.tuple which might be the right way to be.

[jbcoe]

Issue fixed by #500

Follow-up question raised as a new issue in #501