[GHC] #13674: Poor error message which masks occurs-check failure

[GHC]

#13674: Poor error message which masks occurs-check failure
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"c2eea089e7978416c6882a5456117db27b8f45ba/ghc" c2eea08/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="c2eea089e7978416c6882a5456117db27b8f45ba"
 Make isInsolubleOccursCheck more aggressive

 Consider
   type family F a :: * -> *

 Then (a ~ F Int a) is an insoluble occurs check, and can be reported
 as such.  Previous to this patch, TcType.isInsolubleOccursCheck was
 treating any type-family application (including an over-saturated one)
 as unconditionally not-insoluble.

 This really only affects error messages, and then only slightly. I
 tripped over this when investigating  Trac #13674.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13674#comment:11>
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