[GHC] #10423: Can't infer Monad n from (Monad m, m ~ n)

[GHC]

#10423: Can't infer Monad n from (Monad m, m ~ n)
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        Reporter:  Feuerbach         |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.11
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"1189196ce7f064af408c9d16874a4c0b78f3a006/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="1189196ce7f064af408c9d16874a4c0b78f3a006"
 Re-do superclass solving (again); fixes #10423

 TcInstDcls.tcSuperClasses was getting increasingly baroque as a
 succession of tickets (#10423 being the latest) pointed out that
 my cunning plan was not so cunning.

 The big issue is how to restrict the evidence that we generate
 for superclass constraints in an instance declaration to avoid
 superclass loops.  See Note [Recursive superclasses] in TcInstDcls
 which explains the plan.

 The question is how to implement the plan.  The new implementation is
 much neater, and is described in Note [Solving superclass constraints]
 in TcInstDcls.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10423#comment:2>
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