[GHC] #11948: GHC forgets constraints

[GHC]

#11948: GHC forgets constraints
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  typecheck/should_fail/T11948
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"643706e44935cd15c2248e5345dadd3e9804688e/ghc"
 643706e4/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="643706e44935cd15c2248e5345dadd3e9804688e"
 Narrow the warning for simplifiable constraints

 In Trac #11948 I added the warning
     -Wsimplifiable-class-constraints
 which warns if the class constraints in a type signature are
 simplifiable.

 But in fact the fragility it warns about only happens with
 NoMonoLocalBinds, so this patch switches off the warning if
 you have MonoLocalBinds (and suggests using it in the error
 message).

 See Note [Simplifiable given constraints] in TcValidity.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11948#comment:8>
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