The way a Mercurial plugin interacts with Mercurial has changed over time. The technique used in jcheck has been deprecated for a while and in 4.1, it no longer works.

I have a patch that adopts the new method using decorators, introduced in Mercurial 1.9. It also contains a fallback workaround that keeps compatibility with older versions.

I have the same changes ready for defpath which doesn't have it's own component here.