While fiddling with template metaprogramming I managed to run into an
issue related to some std:: containers and type traits. As far as I have
researched it, it is a known (although not obvious at first) shortcoming.
Consider the following code snippet:
using MoveOnly = std::unique_ptr<Copyable>;
(BTW, the vector is used here as an example, but the list and
forward_list also behave the same way for the same reason)
Counter-intuitively, the static assert fails. This stems from the fact
that the vector's copy constructor is not a function template; thus it
cannot be SFINAEd (everything can be a verb if one tries hard enough ;))
away, because it's always declared.
Since C++20, the copy constructor can be constrained:
vector(vector const&) requires std::copyable<T>;
I think this would solve your issue. This would require the containers general requirements to lift this requirement (on `X u(a)`, that `T` is Cpp17CopyInsertable, etc.) from a precondition to a constraint. There are probably quite a few clauses that would need to be considered in such an effort.