[GHC] #11503: TypeError woes (incl. pattern match checker)

[GHC]

#11503: TypeError woes (incl. pattern match checker)
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings,
                                     |  CustomTypeErrors
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14141            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 I ran into this issue a few days ago, and it really is annoying. We're
 forced to choose between a constraint the pattern checker will work with
 properly (`'True ~ 'False`) and a constraint that will give a good error
 message when accidentally matching against `MkT2` (`TypeError ...`).
 There's a sort of workaround, but it's really horrible: instead of `'True
 ~ 'False`, use something like `"" ~ "This is an error message!"`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11503#comment:3>
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