What is the purpose of `unique_impl_ptr(unique_impl_ptr< U, D > &&u)`?

[meator]

I don't understand the purpose of unique_impl_ptr(unique_impl_ptr< U, D > &&u). spimpl seems to care about construction/assignment from not only types identical to T, but also types convertible to it.

What is the use case for this feature? The PIMPLs I know don't use inheritance often.

Also, I have noticed this function in particular because it contains a bug. It tries to access the ptr_ protected member of the u argument. If it's called like this:

spimpl::unique_impl_ptr<BaseType>(spimpl::unique_impl_ptr<DerivedType>());

spimpl::unique_impl_ptr<BaseType> and spimpl::unique_impl_ptr<DerivedType> are not related classes, so they cannot access private nor protected members of each other.

EDIT: spimpl::impl_ptr::impl_ptr(spimpl::impl_ptr::impl_ptr<U, D, C> &&) suffers from the same problem.

EDIT 2: spimpl::impl_ptr::operator=(const spimpl::impl_ptr::impl_ptr<U, D, C> &) seems to lead to different and more subtle errors.

I would have made a PR for this (like I've done before), but I am not sure what would be the best way to fix this.

[meator]

I see that std::unique_ptr has a similar move constructor from a convertible instance of the pointer, so this could have some use.

A solution to this problem could be marking all instantiations of spimpl::unique_impl_ptr friends of each other (like described here). The same could be done for spimpl::impl_ptr.

A impl ptr implementation should only inherit functions of std::unique_ptr which are relevant to PIMPLs. Is having such move constructor relevant for PIMPLs? I'm open to discussion.