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### 2842. `in_place_t` check for `optional::optional(U&&)` should decay `U`

Section: 20.6.3.2 [optional.ctor] Status: C++17 Submitter: Tim Song Opened: 2016-12-13 Last modified: 2020-09-06

Priority: 0

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Discussion:

As `in_place_t` is a normal tag type again, we need to decay `U` before doing the `is_same_v` check.

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4618.

1. Edit 20.6.3.2 [optional.ctor] as indicated:

   >     template <class U = T>
   >       EXPLICIT constexpr optional(U&& v);

   > > […]

   > > -22- Remarks: If `T`'s selected constructor is a `constexpr` constructor, this constructor shall be a `constexpr` constructor. This constructor shall not participate in overload resolution unless `is_constructible_v<T, U&&>` is `true`, `is_same_v<decay_t<U>, in_place_t>` is `false`, and `is_same_v<optional<T>, decay_t<U>>` is `false`. The constructor is explicit if and only if `is_convertible_v<U&&, T>` is `false`.