[GHC] #12068: RULE too complicated to desugar when using constraint synonyms

15 May 2016

      #12068: RULE too complicated to desugar when using constraint synonyms
-------------------------------------+-------------------------------------
           Reporter:  crockeea       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 With the following minimal-as-I-could-get-it example,
 {{{
 {-# LANGUAGE ConstraintKinds #-}

 class Qux a
 class (Num r) => Class1 r
 class (Num r) => Class2 r

 newtype Foo q z = Foo z
 type Qux' q z = (Qux q, Integral z)
 instance (Num z) => Num (Foo q z)
 instance (Qux' q z, Num z) => Class1 (Foo q z)
 instance (Qux' q z, Num z) => Class2 (Foo q z)

 newtype Bar r = Bar r

 {-# SPECIALIZE bar :: (Qux q) => Bar (Foo q Int) -> Bar (Foo q Int) #-}
 bar :: (Class1 r, Class2 r) => Bar r -> Bar r
 bar = undefined
 }}}

 I get the warning:
 {{{
 RULE left-hand side too complicated to desugar
       Optimised lhs: let {
                        $dNum_aGE :: Num (Foo q Int)
                        [LclId, Str=DmdType]
                        $dNum_aGE = Main.$fNumFoo @ q @ Int
 GHC.Num.$fNumInt } in
                      bar
                        @ (Foo q Int)
                        (Main.$fClass1Foo
                           @ q @ Int $dNum_aGE ($dQux_aFH, $dIntegral_aGI)
 GHC.Num.$fNumInt)
                        (Main.$fClass2Foo
                           @ q @ Int $dNum_aGE ($dQux_aFH, $dIntegral_aGI)
 GHC.Num.$fNumInt)
       Orig lhs: let {
                   $dIntegral_aGI :: Integral Int
                   [LclId, Str=DmdType]
                   $dIntegral_aGI = GHC.Real.$fIntegralInt } in
                 let {
                   tup_aGJ :: Qux' q Int
                   [LclId, Str=DmdType]
                   tup_aGJ = ($dQux_aFH, $dIntegral_aGI) } in
                 let {
                   $dNum_aGH :: Num Int
                   [LclId, Str=DmdType]
                   $dNum_aGH = GHC.Num.$fNumInt } in
                 let {
                   $dNum_aGG :: Num Int
                   [LclId, Str=DmdType]
                   $dNum_aGG = $dNum_aGH } in
                 let {
                   tup_aGF :: Qux' q Int
                   [LclId, Str=DmdType]
                   tup_aGF = ($dQux_aFH, $dIntegral_aGI) } in
                 let {
                   $dNum_aGE :: Num (Foo q Int)
                   [LclId, Str=DmdType]
                   $dNum_aGE = Main.$fNumFoo @ q @ Int $dNum_aGH } in
                 let {
                   $dClass2_aFK :: Class2 (Foo q Int)
                   [LclId, Str=DmdType]
                   $dClass2_aFK =
                     Main.$fClass2Foo @ q @ Int $dNum_aGE tup_aGJ $dNum_aGH
 } in
                 let {
                   $dClass1_aFJ :: Class1 (Foo q Int)
                   [LclId, Str=DmdType]
                   $dClass1_aFJ =
                     Main.$fClass1Foo @ q @ Int $dNum_aGE tup_aGF $dNum_aGG
 } in
                 bar @ (Foo q Int) $dClass1_aFJ $dClass2_aFK
 }}}

 This is apparently due to my use of a constraint synonym. In this code it
 would be quite simple to just replace the synonym with the constraints on
 its RHS, but in my real code my constraint synonym is an associated type,
 so that is not an option. It would be great o be able to specialize in the
 presence of constraint synonyms.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12068
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