[GHC] #11581: TypeError requires UndecidableInstances unnecessarily

[GHC]

#11581: TypeError requires UndecidableInstances unnecessarily
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        Reporter:  rwbarton          |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1-rc2
  checker)                           |
      Resolution:                    |             Keywords:
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:"a008eadfaa4816be349b4fefde9b9b9edc1ca359/ghc"
 a008eadf/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="a008eadfaa4816be349b4fefde9b9b9edc1ca359"
 Take type-function arity into account

 ...when computing the size of a call on the RHS of a type
 instance declaration.

 This came up in Trac #11581.  The change is in
    TcType.tcTyFamInsts
 which now trims the type arguments in a call.  See the
 comments with that function definition.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11581#comment:4>
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