[GHC] #15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"

[GHC]

#15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  9123, 14883
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"32eb41994f7448caf5fb6b06ed0678d79d029deb/ghc" 32eb419/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="32eb41994f7448caf5fb6b06ed0678d79d029deb"
 Instances in no-evidence implications

 Trac #15290 showed that it's possible that we might attempt to use a
 quantified constraint to solve an equality in a situation where we
 don't have anywhere to put the evidence bindings.  This made GHC crash.

 This patch stops the crash, but still rejects the pogram.  See
 Note [Instances in no-evidence implications] in TcInteract.

 Finding this bug revealed another lurking bug:

 * An infelicity in the treatment of superclasses -- we were expanding
   them locally at the leaves, rather than at their binding site; see
   (3a) in Note [The superclass story].

   As a consequence, TcRnTypes.superclassesMightHelp must look inside
   implications.

 In more detail:

 * Stop the crash, by making TcInteract.chooseInstance test for
   the no-evidence-bindings case.  In that case we simply don't
   use the instance.  This entailed a slight change to the type
   of chooseInstance.

 * Make TcSMonad.getPendingScDicts (now renamed getPendingGivenScs)
   return only Givens from the /current level/; and make
   TcRnTypes.superClassesMightHelp look inside implications.

 * Refactor the simpl_loop and superclass-expansion stuff in
   TcSimplify.  The logic is much easier to understand now, and
   has less duplication.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15290#comment:18>
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