Inherited restrictions (#2798)

Revamp class modifiers to close loopholes.

Based on Lasse's proposal at: https://gist.github.com/lrhn/981d4528fbb995c23afa0f9436209537

Co-authored-by: Erik Ernst <[email protected]>
Co-authored-by: Lasse R.H. Nielsen <[email protected]>

Author: 3 people