[ufunction] Can't construct ufunction from a free function

[iboB]

This works with msvc, but with clang and gcc it produces:

[build] itlib/include/itlib/ufunction.hpp:106:63: error: invalid use of ‘const_cast’ with type ‘int (&)(int, int)’, which is a pointer or reference to a function type
[build]   106 |         copy_wrapper(const copy_wrapper& other) : func_object(const_cast<FO&&>(other.func_object)) { throw 0; } // should never get to here
[build]       |                                                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Apparently the const_cast fails. However we need this const cast for the fake copy constructor.

Potential solutions would be:

1. Instead of a const cast add a function which works with const_cast for non-free functions and just copies free functions
2. Devise some other way of obtaining a FO instance... something like a code-level declval

The key is that this only needs to compile. It's not important to do something which does sensible stuff when executed.

Not sure how to approach this yet.

[iboB]

Simple repro which can be added to t-ufunction-11.cpp:

int sum(int a, int b) { return a + b; }

TEST_CASE("Func")
{
    itlib::ufunction<int(int,int)> func = sum;
}