Bug with generic args

Made a JIRA for the minor bug I reported in this thread.

JIRA link: [SR-12531] Type-checker fails when determining subclassibility of keypath types · Issue #54974 · apple/swift · GitHub

I'm still not 100% sure this is really a bug; if it's not then please close out the JIRA. However if it's not a bug then it would be nice to know why not.