More comments on role subtyping, unsoundness fix.

Authored by ezyang on Feb 26 2017, 10:15 PM.

Description

More comments on role subtyping, unsoundness fix.

Summary:

  • We incorrectly allowed subroling on injective data in some cases. There is now a test to check for this case, and a Note.
  • More commentary on how the subtyping with roles works.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: goldfire, austin, simonpj, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3222