[GHC] #9939: Warn for duplicate superclass constraints

[GHC]

#9939: Warn for duplicate superclass constraints
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        Reporter:  crockeea          |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"32973bf3c2f6fe00e01b44a63ac1904080466938/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="32973bf3c2f6fe00e01b44a63ac1904080466938"
 Major patch to add -fwarn-redundant-constraints

 The idea was promted by Trac #9939, but it was Christmas, so I did
 some recreational programming that went much further.

 The idea is to warn when a constraint in a user-supplied context is
 redundant.  Everything is described in detail in
   Note [Tracking redundant constraints]
 in TcSimplify.

 Main changes:

  * The new ic_status field in an implication, of type ImplicStatus.
    It replaces ic_insol, and includes information about redundant
    constraints.

  * New function TcSimplify.setImplicationStatus sets the ic_status.

  * TcSigInfo has sig_report_redundant field to say whenther a
    redundant constraint should be reported; and similarly
    the FunSigCtxt constructor of UserTypeCtxt

  * EvBinds has a field eb_is_given, to record whether it is a given
    or wanted binding. Some consequential chagnes to creating an evidence
    binding (so that we record whether it is given or wanted).

  * AbsBinds field abs_ev_binds is now a *list* of TcEvBiinds;
    see Note [Typechecking plan for instance declarations] in
    TcInstDcls

  * Some significant changes to the type checking of instance
    declarations; Note [Typechecking plan for instance declarations]
    in TcInstDcls.

  * I found that TcErrors.relevantBindings was failing to zonk the
    origin of the constraint it was looking at, and hence failing to
    find some relevant bindings.  Easy to fix, and orthogonal to
    everything else, but hard to disentangle.

 Some minor refactorig:

  * TcMType.newSimpleWanteds moves to Inst, renamed as newWanteds

  * TcClassDcl and TcInstDcls now have their own code for typechecking
    a method body, rather than sharing a single function. The shared
    function (ws TcClassDcl.tcInstanceMethodBody) didn't have much code
    and the differences were growing confusing.

  * Add new function TcRnMonad.pushLevelAndCaptureConstraints, and
    use it

  * Add new function Bag.catBagMaybes, and use it in TcSimplify
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9939#comment:2>
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