Fix create instance func

[pupil1337]

To Fix #1425

This PR is a resubmission of #1427 , as the repository submitted that time was my master branch it looked bad. So I want to resubmit it.

[dsnopek]

Thanks!

The goal here sounds great: cause a compilation error if trying to register an abstract class with GDREGISTER_CLASS() (rather than GDREGISTER_ABSTRACT_CLASS()).

However, I think the implementation here will be confusing for developers (the error they get when using the wrong macros doesn't point to the solution) and for maintainers (my brain has trouble with the !std::is_abstract_v<T> || !is_abstract condition :-)).

What if we added another static_assert() to the top of ClassDB::_register_class() instead?

Something like:

	static_assert(std::is_abstract_V<T> && !is_abstract, "Abstract classes must be registered using GDREGISTER_ABSTRACT_CLASS().");

[pupil1337]

Thanks!

The goal here sounds great: cause a compilation error if trying to register an abstract class with GDREGISTER_CLASS() (rather than GDREGISTER_ABSTRACT_CLASS()).

However, I think the implementation here will be confusing for developers (the error they get when using the wrong macros doesn't point to the solution) and for maintainers (my brain has trouble with the !std::is_abstract_v<T> || !is_abstract condition :-)).

What if we added another static_assert() to the top of ClassDB::_register_class() instead?

Something like:

	static_assert(std::is_abstract_V<T> && !is_abstract, "Abstract classes must be registered using GDREGISTER_ABSTRACT_CLASS().");

Thank you for your suggestion, it's very useful! I had change last commit.

[dsnopek]

Thanks!

Two additional notes:

• I think this should be in ClassDB::_register_class() (with a leading underscore) rather than ClassDB::register_class(), because otherwise we have this same problem with GDREGISTER_RUNTIME_CLASS() or GDREGISTER_INTERNAL_CLASS().
• In the error message, we should only recommend the macro (GDREGISTER_CLASS()) not the function. Despite using the functions directly in the test code (which will be fixed soon by PR Use GDREGISTER defines in example #1435) we want to encourage developers to use the macros.

[pupil1337]

• if this class is real a no-abstract class, not check, so it can register by GDREGISTER_CLASS GDREGISTER_VIRTUAL_CLASS GDREGISTER_ABSTRACT_CLASS GDREGISTER_INTERNAL_CLASS GDREGISTER_RUNTIME_CLASS etc.. (all register methods)
• and if this class is real an abstract class, check is_abstract param, make this class must register by GDREGISTER_ABSTRACT_CLASS

[dsnopek]

Thanks, this looks great to me now!

[dsnopek]

Congrats on your first merged Godot contribution 🎉

[pupil1337]

Thanks🌹

[dsnopek]

Cherry-picked for 4.2 in PR #1465

[dsnopek]

Cherry-picked for 4.1 in PR #1466