[GHC] #12823: Inconsistency in acceptance of equality constraints in different forms

[GHC]

#12823: Inconsistency in acceptance of equality constraints in different forms
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        Reporter:  dfeuer            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  GADTs
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I agree it's a bit odd but I don't see how to do better.

 Generally, GHC's view is that the language-extension flags apply to the
 source code you write in the module being compiled; and NOT to the library
 code that you are implicitly using thereby.

 Type synonyms are very transparent, and hide nothing, which is a bit in
 conflict with the above.  Perhaps we could make them opaque for the
 purpose of language-extension tests?  I'm sure we'd get different
 complaints then :-).

 So I don't see a straightforward way to fix this inconsistency.  Yet.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12823#comment:4>
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