[GHC] #11573: CallStacks show up unexpectedly in type of local binding in GHCi

[GHC]

#11573: CallStacks show up unexpectedly in type of local binding in GHCi
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           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.0.1-rc2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 It was noticed (in a thread on ghc-devs) that introducing a CallStack-
 carrying term into the RHS of a let binding in GHCi produces a type which
 itself carries a callstack.

 I could be wrong but as far as I know this is a bug: the solver should
 never introduce a constraint that the user didn't write; it should merely
 provide an empty callstack in the event that a callstack constraint can't
 be solved.

 {{{
 Prelude> let myList = [1, 2, 3 :: Integer]
 Prelude> let myList' = myList ++ undefined
 Prelude> :t myList
 myList :: [Integer]
 Prelude> :t myList'
 myList' :: (?callStack::GHC.Stack.Types.CallStack) => [Integer]
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11573>
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