[GHC] #10845: Incorrect behavior when let binding implicit CallStack object

[GHC]

#10845: Incorrect behavior when let binding implicit CallStack object
-------------------------------------+-------------------------------------
        Reporter:  nitromaster101    |                Owner:  gridaphobe
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10846            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by gridaphobe):

 Alas, we can't simply remove the no-given solver for CallStacks, what
 about

 {{{
 f :: CallStack
 f = ?callStack
 }}}

 Here we're given a type signature, so we won't pass through
 `simplifyInfer`. In general, I'm pretty sure we need the no-given solver
 for CallStacks at levels 1 and 3.

 Unfortunately that doesn't cut it either, consider our friend `fail`

 {{{
 class Monad m where
   fail :: String -> m a
   fail s = error s
 }}}

 `fail` gives rise to an implication

 {{{
   Implic {
     TcLevel = 3
     Skolems = (m_anr[ssk] :: * -> *)
     No-eqs = False
     Status = Unsolved
     Given = $dMonad_aon :: T10845.Monad m_anr[ssk]
     Wanted =
       WC {wc_impl =
             Implic {
               TcLevel = 5
               Skolems = a_aop[sk]
               No-eqs = False
               Status = Unsolved
               Given =
               Wanted =
                 WC {wc_simple =
                       [W] $dIP_aot :: ?callStack::CallStack
 (CNonCanonical)}
               Binds = EvBindsVar<aov>
               the type signature for:
                 T10845.fail :: String -> m_anr[ssk] a_aop[sk] }}
     Binds = EvBindsVar<aow>
     the class declaration for ‘T10845.Monad’ }
 }}}

 which has the CallStack wanted stuck back at level 5! Ugh..

 Perhaps what we really want is to move the no-given solver to
 `simplifyTop`, not `simplifyInfer`. I see that `simplifyTop` handles type-
 class defaulting, which feels similar in spirit to the no-given solver (ie
 we're defaulting the insoluble CallStacks to contain just the current
 source location).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10845#comment:11>
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