constraint deduction bug?

[Dimitrios Vytiniotis]

Is Intersect (or some other of the constructors involved, maybe Empty?) kind-polymorphic and instantiated 
in the two sites (wanted vs. given) with different kind variables which end up being unconstrained, 
or something like that? Then it might be the case that we don't get a match (which is rather unintuitive, I 
agree) and the constraint is not solved.

Even if it's a case like this it's probably best to do submit a small reproducible example and we will have a look.

Thanks
d-



> -----Original Message-----
> From: ghc-devs-bounces at haskell.org [mailto:ghc-devs-
> bounces at haskell.org] On Behalf Of Richard Eisenberg
> Sent: Friday, March 22, 2013 1:11 AM
> To: Gabor Greif
> Cc: ghc-devs at haskell.org
> Subject: Re: constraint deduction bug?
> 
> It certainly looks trivial to me.
> 
> Richard
> 
> On Mar 21, 2013, at 7:59 PM, Gabor Greif <ggreif at gmail.com> wrote:
> 
> > In context of a bigger source file I got this error from GHC (HEAD)
> >
> >    Could not deduce (Intersect
> >                        [KeySegment] (BuriedUnder sub k ('Empty
> > [KeySegment])) inv
> >                      ~ 'Empty [KeySegment])
> >    from the context (Intersect
> >                        [KeySegment] (BuriedUnder sub k ('Empty
> > [KeySegment])) inv
> >                      ~ 'Empty [KeySegment])
> >      bound by a pattern with constructor
> >                 Nil :: forall (k :: BOX). Sing (Inventory k) ('Empty k),
> >               in a pattern binding in
> >                    'do' block
> >
> > Can somebody confirm that this constraint should be trivially
> > deducible from the context?
> >
> > Then I'll try to come up with a reduced repro case.
> >
> > Cheers,
> >
> > 	Gabor
> >
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> 
> 
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