[GHC] #12742: Instantiation of invisible type family arguments is too eager

[GHC]

#12742: Instantiation of invisible type family arguments is too eager
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           Reporter:  goldfire       |             Owner:  goldfire
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This module looks, to me, like it should be accepted:

 {{{
 {-# LANGUAGE TypeInType, RankNTypes, TypeFamilies #-}

 module Bug where

 import Data.Kind

 type family F :: forall k2. (k1, k2)

 data T :: (forall k2. (Bool, k2)) -> Type

 type S = T F
 }}}

 But it's not. The problem is that `TcHsType.handle_tyfams` eagerly
 instantiates ''all'' invisible arguments to a type family at every
 occurrence. Instead, it should instantiate only those that are counted in
 the TF's arity -- that is, those "before the colon".

 Will fix.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12742>
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