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

[GHC]

#10845: Incorrect behavior when let binding implicit CallStack object
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        Reporter:  nitromaster101    |                   Owner:
            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:                    |  Differential Revisions:
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Comment (by nitromaster101):

 There are two issues: since f3 doesn't produce any warnings, neither
 should f since the only difference is the type of ?loc.

 If I add some callers to f and f2:

 {{{
 {-# LANGUAGE ImplicitParams #-}

 import GHC.Types

 f :: (?loc :: CallStack) => String
 f = let y = show $ map (srcLocStartLine . snd) $ getCallStack ?loc
     in y

 f2 :: (?loc :: CallStack) => String
 f2 = show $ map (srcLocStartLine . snd) $ getCallStack ?loc

 f_caller = f
 f2_caller = f2
 }}}

 We get the warning:
 {{{
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 Bug.hs:5:6: warning:
     Redundant constraint: ?loc::CallStack
     In the type signature for:
          f :: (?loc::CallStack) => String
 }}}

 And when we run them the call stack returned by f is wrong, it should have
 two entries (the line number for f_caller and f):
 {{{
 *> f_caller
 "[6]"
 *> f2_caller
 "[10,13]"
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10845#comment:2>
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