Rationale:

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

[ 2020-05-29; Billy Baker comments ]

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

[ 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. 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.

[ 2017-01-27 Telecon ]

Priority 2

LWG 2756 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.