Richard is right.

Let’s attach this useful info to the ticket, rather than ghc-devs.   I’ve done that for this exchange.

https://ghc.haskell.org/trac/ghc/ticket/10789#comment:18

Simon

From: ghc-devs <ghc-devs-bounces@haskell.org> On Behalf Of Richard Eisenberg
Sent: 18 June 2018 04:21
To: sasa bogicevic <brutallesale@gmail.com>
Cc: ghc-devs@haskell.org
Subject: Re: #10789

On Jun 17, 2018, at 10:01 AM, sasa bogicevic <brutallesale@gmail.com> wrote:

So I guess that we need to check if one of the kinds of two types we are comparing defaults to * (or  Type if you will) and then

add new warning that will be more descriptive as to why the failure happened. Maybe there is a way to check if what we are

comparing are actually type families so that would make the job easier I guess.

I don't think the problem is particular to `Type` or defaulting. Instead, the problem is when one of the two mismatched types is a type family application where the type family has equations that pattern-match on an invisible parameter, and it's that invisible-parameter matching that's gone awry. Now that I think about it, detecting these particular conditions might be tricky: you might need to edit code in FamInstEnv that does type family equation lookup to return diagnostic information if a match fails. (I would look at reduceTyFamApp_maybe, and perhaps it can return something more interesting than Nothing in the failure case.) 

Richard Eisenberg offered some help on this but I am not sure how to grab hold of him so I'd appreciate any help I could get.

Just email! :)

Thanks for looking into this!

Richard