[GHC] #14040: Typed holes regression in GHC 8.0.2: No skolem info: z_a1sY[sk:2]

[GHC]

#14040: Typed holes regression in GHC 8.0.2: No skolem info: z_a1sY[sk:2]
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.2
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies, PartialTypeSignatures
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13877            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 OK I understand the problem.  Consider the type signature (abbreviated
 from the one written)
 {{{
 -- Sing :: forall (a:*). WeirdList a -> *

 elimWeirdList :: forall (a :: Type) (wl :: WeirdList a)
                         (p :: forall (x :: Type). x -> WeirdList x ->
 Type).
                  Sing wl
                  -> (forall (z :: Type) (x :: z) (xs :: WeirdList
 (WeirdList z)).
                        Sing x -> Sing xs -> p _ xs
                                             -- p @ (WeirdList z) (_ ::
 WeirdList z) xs
                      -> p _ (WeirdCons x xs))
                  -> p _ wl
 }}}
 I have written ou the explicitly kind application needed in the
 application `(p _ xs)`.

 Now remember that a signature with wildcards is like a template imposed on
 type inference.  The `_` wildcards are replaced with unification
 variables.  So it's like this
 {{{
 alpha1 :: kappa1
 alpha2 :: kappa2
 alpha3 :: kappa3

 elimWeirdList :: forall (a :: Type) (wl :: WeirdList a)
                         (p :: forall (x :: Type). x -> WeirdList x ->
 Type).
                  Sing wl
                  -> (forall (z :: Type) (x :: z) (xs :: WeirdList
 (WeirdList z)).
                        Sing x -> Sing xs -> p alpha1 xs
                                             -- p @ (WeirdList z) (alpha1
 :: WeirdList z) xs
                      -> p alpha2 (WeirdCons x xs))
                  -> p alpah3 wl
 }}}
 These unification variables are at the "top" of the type.  Once you see
 this it is clear that simply kind-checking this type should fail, becuase
 `alpha` must have kind `WeirdList z`, and `z` is bound by that inner
 forall. Yikes!

 This is all happening because we aren't generating an implication
 constraint when we kind-check a forall.  In fact, there are other symptoms
 of the same problem: see #14066.  Let's cure that first, and then come
 back to this one (which might "just work").

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14040#comment:1>
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