[GHC] #13530: Horrible error message due to TypeInType

[GHC]

#13530: Horrible error message due to TypeInType
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeInType
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 Simon Peyton Jones <simonpj@…>):

 In [changeset:"bac95f9de5bd8d0a647a3a1e4492497603c2fda2/ghc"
 bac95f9d/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="bac95f9de5bd8d0a647a3a1e4492497603c2fda2"
 Yet another attempt at inferring the right quantification

 TcSimplify.decideQuantification is truly a tricky function!
 Trac #13509 showed that we were being over-eager with defaulting
 of runtime-rep variables (levity polymorphism), which meant that
 a program was wrongly rejected, and with a very odd error message
 (c.f. Trac #13530)

 I spent an unreasonably long time figuring out how to fix this
 in a decent way, and ended up with a major refactoring of
 decideQuantification, with a kock-on effect in simplifyInfer.

 It is at least a bit more comprehensible now; but I still
 can't say I like it.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13530#comment:4>
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