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2736. nullopt_t insufficiently constrained

Section: 22.5.4 [optional.nullopt] Status: C++17 Submitter: Tim Song Opened: 2016-06-17 Last modified: 2017-07-30

Priority: 2

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Discussion:

22.5.4 [optional.nullopt]/2 requires of nullopt_t that

Type nullopt_t shall not have a default constructor. It shall be a literal type. Constant nullopt shall be initialized with an argument of literal type.

This does not appear sufficient to foreclose the following implementation:

struct nullopt_t 
{
  constexpr nullopt_t(const nullopt_t&) = default;
};

constexpr nullopt_t nullopt(nullopt_t{});

• nullopt_t has no default constructor because it has a user-declared (copy) constructor;
• nullopt_t has a trivial destructor, is an aggregate, and is a literal type;
• nullopt has been initialized with an argument of literal type, to wit, nullopt_t.

But such a nullopt_t is still constructible from {} and so still makes opt = {} ambiguous.

[2016-08 Chicago]

This is related to LWG 2510⁽ⁱ⁾.

Monday PM: Ville to provide updated wording

Fri AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Edit 22.5.4 [optional.nullopt]/2 as indicated:

   [Drafting note: {} can do one of three things for a class type: it may be aggregate initialization, it may call a default constructor, or it may call an initializer-list constructor (see 9.4.5 [dcl.init.list], 12.2.2.8 [over.match.list]). The wording below forecloses all three possibilities. — end drafting note]

   -2- Type nullopt_t shall not have a default constructor or an initializer-list constructor. It shall not be an aggregate and shall be a literal type. Constant nullopt shall be initialized with an argument of literal type.