RULE too complicated to desugar

Another example of specialization failing. Unlike #12068 (closed), this example doesn't use constraint/type synonyms.

{-# LANGUAGE FlexibleContexts #-}

data Bar a
instance (Num a) => Num (Bar a)

data Foo q
instance (C1 q) => Num (Foo q)

class C1 a

class (Num r, Num (Bar r)) => C2 r
instance (C1 q) => C2 (Foo q)
instance (C2 r) => C2 (Bar r)

{-# SPECIALIZE f :: (C1 q) => Foo q -> Foo q #-}
f :: (C2 r, C2 (Bar r)) => r -> r
f = undefined

Warning:

RULE left-hand side too complicated to desugar
      Optimised lhs: let {
                       $dNum_aFp :: Num (Foo q)
                       [LclId, Str=DmdType]
                       $dNum_aFp = Main.$fNumFoo @ q $dC1_aEj } in
                     let {
                       $dNum_aFq :: Num (Bar (Foo q))
                       [LclId, Str=DmdType]
                       $dNum_aFq = Main.$fNumBar @ (Foo q) $dNum_aFp } in
                     f @ (Foo q)
                       $dC2_aEl
                       (Main.$fC2Bar
                          @ (Foo q)
                          $dNum_aFq
                          (Main.$fNumBar @ (Bar (Foo q)) $dNum_aFq)
                          $dC2_aEl)
      Orig lhs: let {
                  $dNum_aFp :: Num (Foo q)
                  [LclId, Str=DmdType]
                  $dNum_aFp = Main.$fNumFoo @ q $dC1_aEj } in
                let {
                  $dNum_aFq :: Num (Bar (Foo q))
                  [LclId, Str=DmdType]
                  $dNum_aFq = Main.$fNumBar @ (Foo q) $dNum_aFp } in
                let {
                  $dNum_aFr :: Num (Bar (Bar (Foo q)))
                  [LclId, Str=DmdType]
                  $dNum_aFr = Main.$fNumBar @ (Bar (Foo q)) $dNum_aFq } in
                let {
                  $dC2_aEl :: C2 (Foo q)
                  [LclId, Str=DmdType]
                  $dC2_aEl = Main.$fC2Foo @ q $dNum_aFp $dNum_aFq $dC1_aEj } in
                let {
                  $dC2_aEm :: C2 (Bar (Foo q))
                  [LclId, Str=DmdType]
                  $dC2_aEm = Main.$fC2Bar @ (Foo q) $dNum_aFq $dNum_aFr $dC2_aEl } in
                f @ (Foo q) $dC2_aEl $dC2_aEm

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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