[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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Comment (by AntC):

 Replying to [comment:6 sheaf]:
 > What about the following: ...
 >
 > This seems useful to me, ...

 Sure, it's the same structure as the O.P.
 >
 > I feel like the injectivity annotation should be allowed in this case
 ...

 It is. Ryan's comment:5 says what's not allowed is the `TypeError` catch-
 all equation -- that is, if you want `ToDim` to be injective.

 So if you call `ToDim 5`, inference will get stuck, and you'll get an
 error message somewhere/somehow else.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12119#comment:8>
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