realToFrac into Complex Double has no specialization

Example:

module Foo where
import Data.Complex

f :: Double -> Complex Double
f x = realToFrac x

ghc -O -ddump-simpl gives:

f =
  \ (w_s17v :: Double) ->
    case w_s17v of _ [Occ=Dead] { GHC.Types.D# ww1_s17y ->
    case GHC.Float.$w$ctoRational ww1_s17y
    of _ [Occ=Dead] { (# ww3_a152, ww4_a153 #) ->
    case GHC.Float.rationalToDouble ww3_a152 ww4_a153
    of dt_a15c { GHC.Types.D# ipv_a15g ->
    Data.Complex.:+ @ Double dt_a15c Data.Complex.$fFloatingComplex1
    }
    }
    }

This means the conversion goes through Rational. However, in this case, the conversion could be done much more efficiently with (:+0). It would be nice if this happened automatically.

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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