[GHC] #9973: Erroneous Redundant constraint warning

9 Jan 2015


      #9973: Erroneous Redundant constraint warning
-------------------------------------+-------------------------------------
              Reporter:  alanz       |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.11
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 Using version 4425ab99d6410839fa7567950b0a4696b0a3d70f with D538 applied,

 If the attached source file is loaded as is into ghci it generates the
 following warning for line 5, but otherwise loads fine

 {{{
 /home/alanz/mysrc/github/alanz/HaRe/Bug2.hs:5:18: Warning:
     Redundant constraint: SYB.Data t
     In the type signature for:
        duplicateDecl :: SYB.Data t =>
                         [GHC.LHsBind GHC.Name]
                         -> t -> GHC.Name -> GHC.Name -> IO [GHC.LHsBind
 GHC.Name]
 }}}

 If the constraint is commented out, it results in an error

 {{{
 /home/alanz/mysrc/github/alanz/HaRe/Bug2.hs:26:16:
     No instance for (SYB.Data t)
       arising from a use of ‘definingSigsNames’
     Possible fix:
       add (SYB.Data t) to the context of
         the type signature for:
         duplicateDecl :: [GHC.LHsBind GHC.Name]
                          -> t -> GHC.Name -> GHC.Name -> IO [GHC.LHsBind
 GHC.Name]
     In the expression: definingSigsNames [n] sigs
     In an equation for ‘typeSig’: typeSig = definingSigsNames [n] sigs
     In an equation for ‘duplicateDecl’:
         duplicateDecl decls sigs n newFunName
           = do { let sspan = ...;
                  newSpan <- case typeSig of {
                               [] -> return sspan
                               _ -> ... };
                  let rowOffset = ...;
                  .... }
           where
               typeSig = definingSigsNames [n] sigs
 Failed, modules loaded: none.
 }}}

 This behaviour goes away if the following lines are commented out

 {{{#!hs
      let rowOffset = case typeSig of
                         [] -> 2
                         _  -> 1
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9973
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