[GHC] #11715: Constraint vs *

[GHC]

#11715: Constraint vs *
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        Reporter:  bgamari           |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |             Keywords:  Typeable,
      Resolution:                    |  LevityPolymorphism, Roles
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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Comment (by adamgundry):

 I'm inclined to agree with the proposal to use `coreView` in class/family
 instances, so we would regard an instance of `Typeable * Constraint` as
 overlapping `Typeable * Type` and hence not be able to solve the former.

 FWIW, here's an actual implementation of `unsafeCoerce` in GHC 8.0.2
 exploiting this bug:
 {{{#!hs
 {-# LANGUAGE AllowAmbiguousTypes, TypeFamilies, TypeOperators #-}

 import Data.Kind
 import Data.Typeable

 type family F x a b
 type instance F Type       a b = a
 type instance F Constraint a b = b

 foo :: x :~: y -> F x a b -> F y a b
 foo Refl = id

 unsafeCoerce :: a -> b
 unsafeCoerce x = case eqT :: Maybe (Type :~: Constraint) of
                    Nothing -> error "No more bug!"
                    Just r  -> foo r x
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11715#comment:71>
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