[GHC] #15710: Should GHC accept a type signature that needs coercion quantification?

[GHC]

#15710: Should GHC accept a type signature that needs coercion quantification?
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by simonpj:

Old description:

> Consider
> {{{
> f :: forall k (f :: k) (x :: k1). (k ~ (k1 -> *)) => f x
> f = error "uk"
> }}}
> Should we accept it?  Now that we have coercion quantification (Trac
> #15497), I think the answer should be yes, with the elaborated signature
> being
> {{{
> f :: forall k (f::k) (x::k1). forall (co :: k ~# (k1->*)). (f |> co) x
> }}}
> But there is a problem: the user wrote `k ~ (k1 -> *)`, and that's a
> boxed value that  we can't take apart in types.  I'm not sure what to do
> here.

New description:

 Consider
 {{{
 f :: forall k (f :: k) (x :: k1). (k ~ (k1 -> *)) => f x
 f = error "uk"
 }}}
 Should we accept it?  Now that we have coercion quantification (Trac
 #15497), I think the answer should be yes, with the elaborated signature
 being
 {{{
 f :: forall k (f::k) (x::k1). forall (co :: k ~# (k1->*)). (f |> co) x
 }}}
 But there is a problem: the user wrote `k ~ (k1 -> *)`, and that's a boxed
 value that  we can't take apart in types.  I'm not sure what to do here.

 These thoughts arose when contemplating `Note [Emitting the residual
 implication in simplifyInfer]` in `TcSimplify`; see comment:5 in #15497

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15710#comment:7>
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