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### 3196. `std::optional<T>` is ill-formed is `T` is an array

Section: 22.5.3 [optional.optional] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-03-18 Last modified: 2021-02-25

Priority: 0

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Discussion:

22.5.3 [optional.optional] appears to allow arrays:

> `T` shall be an object type other than cv `in_place_t` or cv `nullopt_t` and shall satisfy the Cpp17Destructible requirements (Table 30).

But instantiating it fails, because `value_or` attempts to return `T` by value, which isn't possible for an array type.

Existing practice seems to be to reject array types. Libstdc++ and libc++ give an error for the signature of `value_or`, and MSVC fails a static assert saying the type needs to be destructible (which is misleading, because `int[2]` is destructible, but either way it's ill-formed). Previous resolution [SUPERSEDED]:

> This wording is relative to N4810.

> 1. Modify 22.5.3 [optional.optional] as indicated:

>    > -3- `T` shall be an non-array object type other than cv `in_place_t` or cv `nullopt_t` and shall satisfy the Cpp17Destructible requirements (Table 30).

[2019-03-26 Marshall provides updated resolution based on reflector discussion]

[2019-06-16 Moved to "Tentatively Ready" based on five positive votes on the reflector]

Proposed resolution:

This wording is relative to N4810.

1. In 16.4.4.2 [utility.arg.requirements], edit Table 30 — "Cpp17Destructible requirements" as indicated:

   > Table 30 — Cpp17Destructible requirements
   > Expression | Post-condition
   > --- | ---
   > `u.~T()` | All resources owned by `u` are reclaimed, no exception is propagated.
   > [Note: Array types and non-object types are not Cpp17Destructible. — end note]

1. Modify 22.5.3 [optional.optional] as indicated:

   > -3- `T` shall be a object type other than cv `in_place_t` or cv `nullopt_t` and shall satisfy that meets the Cpp17Destructible requirements (Table 30).

1. Modify 22.6.3 [variant.variant] as indicated:

   > -2-All types in `Types` shall be (possibly cv-qualified) object types that are not arrays meet the Cpp17Destructible requirements (Table 30).