I like the proposal and mostly agree with Nick.
One point I would like to add is to allow for decorators in extension methods.
I am expecting decorators to be very useful in typical web application frameworks and having extensions also support them would be great for having a clean code organization.
This is of course assuming that Mojo will open up custom decorators.
Another point is on the orphan rule. It is very limiting. If we can find solutions to avoid the `spooky effect at a distance` that will be great. May be some way to explicitly resolve the applied extensions at the import time. Need to think about it a bit more deeply…
P.S.
I also prefer `requires T is Copyable` syntax over the colon.