OK, I made #16684 to track the issue: https://gitlab.haskell.org/ghc/ghc/issues/16684 Perhaps we should continue the discussion over there. -Iavor On Tue, May 21, 2019 at 10:18 PM Nicolas Frisby wrote:

I'd be happy to raise a ticket, but it might take a few days -- work trip.

On Tue, May 21, 2019, 01:55 Simon Peyton Jones wrote:

...

We have to be careful in how we define "equality" in the above sentence, including class constraints that (may) have superclass equality constraints.

Indeed. That’s what happens now.

I do think this would work.

Cool. Nick or Iavor: would you like to turn this conversation into a ticket?

(Although it is technically user-facing, it is a very small corner and I’m not sure it would need a GHC proposal – others may want to comment.)

Simon

From: Richard Eisenberg Sent: 21 May 2019 09:43 To: Simon Peyton Jones Cc: Nicolas Frisby ; Iavor Diatchki ; ghc-devs@haskell.org; Ryan Scott Subject: Re: Bug or feature?

On May 21, 2019, at 10:23 AM, Simon Peyton Jones via ghc-devs wrote:

But (A) looks sound to me.

I like (A). (B) makes me nervous, too.

...

A. An implication is considered to “bind local equalities” iff it has at least one given equality whose free variables are not all bound by the same implication.

We have to be careful in how we define "equality" in the above sentence, including class constraints that (may) have superclass equality constraints. I do think this would work.

Richard