[GHC] #15076: Typed hole with higher-rank kind causes GHC to panic (No skolem info)

[GHC]

#15076: Typed hole with higher-rank kind causes GHC to panic (No skolem info)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |             Keywords:  TypedHoles,
      Resolution:                    |  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14040, #14880    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 This is another example of #14880

 The problem in comment:5 comes because when we kind-check foo's type
 signature, we generate this:
 {{{
 foo :: forall {xx :: a}.
        forall (a :: Type) (f :: forall (x::a). Proxy @Type x -> Type)
        Proxy @(Proxy @a xx -> Type)
              (f @xx)
        -> ()
 }}}
 Note that the `f` in `Proxy f` gets elaborated to `Proxy @.. (f @xx)`;
 that is `f` is instantiated with a fresh unification variable `xx`.

 But now we kind-generalise the result and put `xx` at the front.  But
 `xx`'s kind mentions `a` -- disaster.

 Solution proposed for #14880: do not generalise over `xx`; instead default
 it to `Any`. So we'd get
 {{{
 foo :: forall {xx :: a}.
        forall (a :: Type) (f :: forall (x::a). Proxy @Type x -> Type)
        Proxy @(Proxy @a (Any a) -> Type)
              (f @(Any a))
        -> ()
 }}}
 which is probably fine.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15076#comment:6>
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