[GHC] #12119: Can't create injective type family equation with TypeError as the RHS

[GHC]

#12119: Can't create injective type family equation with TypeError as the RHS
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  feature request   |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:  wontfix           |  CustomTypeErrors, TypeFamilies,
                                     |  InjectiveFamilies
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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Changes (by sheaf):

 * cc: sheaf (added)


Comment:

 I understand that the type family is injective if I remove the type error.
 However I would prefer a custom error message, as opposed to the user
 getting some confusing error about stuck type families.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12119#comment:9>
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