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2825. LWG 2756 breaks class template argument deduction for optional

Section: 22.5.3 [optional.optional] Status: Resolved Submitter: Richard Smith Opened: 2016-11-24 Last modified: 2020-11-09

Priority: 2

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Discussion:

LWG 2756⁽ⁱ⁾ applies these changes:

constexpr optional(const T&);
constexpr optional(T&&);
template <class... Args> constexpr explicit optional(in_place_t, Args&&...);
template <class U, class... Args>
  constexpr explicit optional(in_place_t, initializer_list<U>, Args&&...);
template <class U = T> EXPLICIT constexpr optional(U&&);
template <class U> EXPLICIT optional(const optional<U>&);
template <class U> EXPLICIT optional(optional<U>&&);

These break the ability for optional to perform class template argument deduction, as there is now no way to map from optional's argument to the template parameter T.

[2017-01-27 Telecon]

Priority 2

[2017-01-30 Ville comments:]

Seems like the problem is resolved by a simple deduction guide:

template <class T> optional(T) -> optional<T>;

The paper p0433r0 seems to suggest a different guide,

template<class T> optional(T&& t) -> optional<remove_reference_t<T>>;

but I don't think the paper is up to speed with LWG 2756⁽ⁱ⁾. There's no reason to use such an universal reference in the guide and remove_reference in its target, just guide with T, with the target optional<T>, optional's constructors do the right thing once the type has been deduced.

[2020-05-29; Billy Baker comments]

At Kona 2017, P0433R2 was accepted and added Ville's deduction guide rather than one proposed in P0433R0.

[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]

Rationale:

Proposed resolution: