[GHC] #11348: Local open type families instances ignored during type checking

[GHC]

#11348: Local open type families instances ignored during type checking
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        Reporter:  alexvieth         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             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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Comment (by alexvieth):

 > Why should the instance declarations affect the kind of the type
 constructor?

 We could always find type families mentioned in GADT data constructors,
 but with GADT data constructor promotion, we can therefore find these
 families mentioned in kinds:

 {{{#!hs
 type family TrivialFamily (t :: Type) :: Type
 type instance TrivialFamily t = Bool

 data Q where
     Q :: TrivialFamily Int -> Q

 -- The type of the data constructor
 Q :: Bool -> Q

 -- The kind of the promoted data constructor.
 -- GHC actually gives 'Q :: TrivialFamily Int -> Q
 -- but it *should* flatten that to Bool.
 'Q :: Bool -> Q

 -- This will not kind-check, unless we import the
 -- TrivialFamily t instance.
 type Problem = 'Q 'True
 }}}

 To check that a use of `'Q` is well-kinded, we must know `TrivialFamily
 Int`. Pardon my earlier comment, this bug can arise ''without'' switching
 on `TypeInType`.

 > If you're up to this task, that would be wonderful.

 Absolutely. This bug is blocking a project of mine so I want it fixed
 asap.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11348#comment:4>
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