[GHC] #15683: coerce fails for Coercible type families

[GHC]

#15683: coerce fails for Coercible type families
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.6.1
           Keywords:  TypeFamilies   |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I want to bring attention to this
 [https://www.reddit.com/r/haskell/comments/9io4xw/coercing_type_families_when_type_instances_are/
 reddit post] that boils down to

 {{{#!hs
 type family   X a
 type instance X Int  = String
 type instance X Bool = String

 data T a = T (X a)
 }}}

 but not being able to coerce

 {{{#!hs
 coerce :: T Int -> T Bool
 }}}

 This gives the error “Couldn't match type ‘`Int`’ with ‘`Bool`’ arising
 from a use of ‘`coerce`’”.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15683>
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