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

[GHC]

#11503: TypeError woes (incl. pattern match checker)
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        Reporter:  goldfire          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
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 simonpj):

 * keywords:   => PatternMatchWarnings


Comment:

 I think you are saying that
 {{{
 type family NotInt a where
   NotInt Int = False ~ True
   NotInt _   = (() :: Constraint)
 }}}
 makes everything work. That's because the pattern-match warning
 infrastructure treats `False ~ True` as definitely insoluble.

 But apparently it doesn't treat `TypeErorr blah` as definitely insoluble.
 That's because when we solve `[G] NotInt Int`, we get `[G] TypeError ... ~
 fsk, [G] fsk`, and neither is treated as insoluble.

 I'm sure this can be dealt with, but it'll need a little care.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11503#comment:1>
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