optional is "is_copy_constructible" when T is not.

[biocomp]

Hi.
I encountered issues while trying to use std::vector<boost::optional<std::tuple<NonCopyable, std::function<...>>>. THis failed to compile on msvc.

Looks like it boiled down to this repro: https://godbolt.org/g/cFNcUN

#include <boost/optional.hpp>
#include <type_traits>
#include <optional>

struct NonCopyAble
{
    NonCopyAble() = default;

    NonCopyAble(const NonCopyAble&) = delete;
    NonCopyAble& operator=(const NonCopyAble&) = delete;

    NonCopyAble(NonCopyAble&&) = default;
    NonCopyAble& operator=(NonCopyAble&&) = default;
};

static_assert(!std::is_copy_constructible<NonCopyAble>::value, "NonCopyAble is_copy_constructible when shouldn't be");
static_assert(!std::is_copy_constructible<std::optional<NonCopyAble>>::value, "std::optional<NonCopyAble> is_copy_constructible when shouldn't be");    
static_assert(!std::is_copy_constructible<boost::optional<NonCopyAble>>::value, "boost::optional<NonCopyAble> is_copy_constructible when shouldn't be");    // This assert fails

[akrzemi1]

Thanks for the report. I will investigate. I acknowledge the divergence between std::optional and boost::optional. But in general the expectation on transitive detectable copiability is not valid:

static_assert(!std::is_copy_constructible<std::vector<NonCopyAble>>::value,
              "vector of this is not copyable");

[joergbrech]

I encountered the same issue with std::is_copy_assignable, see here: https://stackoverflow.com/q/78157267/12173376