[GHC] #11248: Ambiguous Types with Constraint Synonyms

[GHC]

#11248: Ambiguous Types with Constraint Synonyms
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  polykinds/T11248
      Blocked By:                    |             Blocking:
 Related Tickets:  #11330            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"3c060f36f6eb4d359f252168e2f97b573d017080/ghc"
 3c060f36/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="3c060f36f6eb4d359f252168e2f97b573d017080"
 Fix exprIsHNF (Trac #11248)

 Blimey!  CoreUtils.exprIsHNFlike had not one but two bugs.

  * is_hnf_like treated coercion args like type args
    (result: exprIsHNF might wrongly say True)

  * app_is_value treated type args like value args
    (result: exprIsHNF might wrongly say False)

 Bizarre.  This goes back to at least 2012. It's amazing that it
 hasn't caused more trouble.

 It was discovered by a Lint error when compiling Trac #11248 with -O.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11248#comment:8>
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