Simon Peyton Jones authored Oct 29, 2019 and Marge Bot committed Nov 01, 2019

This patch fixes #17395, a very subtle and hard-to-trigger
bug in tcMatchTy.  It's all explained in
  Note [Matching in the presence of casts (2)]

I have not added a regression test because it is very hard
to trigger it, until we have the upcoming mkAppTyM patch,
after which lacking this patch means you can't even compile
the libraries.