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
<[email protected]> 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 <[email protected]> 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 <[email protected]>
>> Sent: 21 May 2019 09:43
>> To: Simon Peyton Jones <[email protected]>
>> Cc: Nicolas Frisby <[email protected]>; Iavor Diatchki 
>> <[email protected]>; [email protected]; Ryan Scott 
>> <[email protected]>
>> Subject: Re: Bug or feature?
>>
>>
>>
>>
>>
>>
>>
>> On May 21, 2019, at 10:23 AM, Simon Peyton Jones via ghc-devs 
>> <[email protected]> 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
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