EDITED: After quite a bit of discussion, a proposal based off of the description below (but with some different choices in syntax) has been accepted here.

Users have asked for the ability to prevent a class from being extended (#987) and/or implemented (#704). For exhaustiveness checks in pattern matching, we also need the ability to define a type with a known closed set of subtypes.

The Type Modifiers proposal addresses those. It's an alternative to the earlier Packaged Libraries proposal. The very brief summary is:

• Remove the ability to use a class as a mixin (Do not allow mixing in a class, with migration path #1643).
• Types default to fully permissive as they are today.
• Use closed on a class to disable extending it.
• Use base on a class or mixin to disables its implicit interface.
• Use a switch modifier on a class or mixin to defines the root of a sealed type family for exhaustiveness checking. It also implies abstract.

This issue is to discuss the proposal. Feel free to file other more specific issues too.