Fixed box construction error by excluding array_impl type

[aimoonchen]

No description provided.

[aimoonchen]

test code:

#include <immer/array.hpp>
#include <immer/box.hpp>
#include <immer/array_transient.hpp>
struct Value {
    using Array = immer::array<immer::box<Value>>;
    Value() = default;
    Value(immer::box<int>) {}
    Value(immer::box<double>) {}
    Value(Array) {}
};
void test()
{
    Value::Array{}.transient().persistent();
}

[alex-sparus]

Thank you for your contribution!

Unfortunately, this check breaks the forward declared types. Have a look: 6d22844

Would be nice to have both though! Let us know if you come up with a solution

[aimoonchen]

@alex-sparus Thank you for your quick response, how about the new solution?

[arximboldi]

Superseded by: #318