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Create an exposition-only helper for the std::optional copy/move ctor constraints #5205

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jwakely opened this issue Jan 14, 2022 · 0 comments · Fixed by #5416
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Create an exposition-only helper for the std::optional copy/move ctor constraints #5205

jwakely opened this issue Jan 14, 2022 · 0 comments · Fixed by #5416
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jwakely commented Jan 14, 2022 •
edited

In [optional.ctor] optional(const optional&) has:

Constraints:

  • is_­constructible_­v<T, const U&> is true,
  • is_­constructible_­v<T, optional<U>&> is false,
  • is_­constructible_­v<T, optional<U>&&> is false,
  • is_­constructible_­v<T, const optional<U>&> is false,
  • is_­constructible_­v<T, const optional<U>&&> is false,
  • is_­convertible_­v<optional<U>&, T> is false,
  • is_­convertible_­v<optional<U>&&, T> is false,
  • is_­convertible_­v<const optional<U>&, T> is false, and
  • is_­convertible_­v<const optional<U>&&, T> is false.

Then we do the same thing for optional(optional&&), only varying the first bullet.

We're also adding more of this in the std::expected paper, so something like an exposition-only variable template that could be used repeatedly would be good.
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[optional.ctor], [expected.object.ctor] Add converts-from-any-cvref jwakely/draft
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