GHC does not inline `timesDouble` into lambda.

This happens in 8.6.4 as well as GHC-HEAD.

In nofib/imaginary/integrate
we have this function:

zark u v = integrate2D 0.0 u 0.0 v (\x->(\y->x*y))

Which really is zark u v = integrate2D 0.0 u 0.0 v (\x->(\y->x ´timesDouble´ y))

Where we should inline timesDouble into the lambda. Instead we get a closure that calls timesDouble.

I can see why we can't get rid of the closure, but the overhead of a call here seems not worth it.

Relevant core:

Main.main_go2
  = \ (ds_a3Zb :: [Double]) (ds1_a3Zc :: [Double]) ->
      case ds_a3Zb of {
        [] -> GHC.Types.[] @ Double;
        : ipv_a3Zg ipv1_a3Zh ->
          case ds1_a3Zc of {
            [] -> GHC.Types.[] @ Double;
            : ipv2_a3Zm ipv3_a3Zn ->
              GHC.Types.:
                @ Double
                (case ipv2_a3Zm of { GHC.Types.D# ww1_s5S2 ->
                 case $wintegrate1D_r5YG
                        0.0##
                        ww1_s5S2
                        (\ (y_a1wV :: Double) ->
                           case ipv_a3Zg of { GHC.Types.D# ww3_X5T5 ->
                           case $wintegrate1D_r5YG
                                  0.0##
                                  ww3_X5T5
                                  (\ (x_a1wW :: Double) -> GHC.Float.timesDouble x_a1wW y_a1wV)
                           of ww4_s5S6
                           { __DEFAULT ->
                           GHC.Types.D# ww4_s5S6
                           }
                           })
                 of ww2_s5S6
                 { __DEFAULT ->
                 GHC.Types.D# ww2_s5S6
                 }
                 })
                (Main.main_go2 ipv1_a3Zh ipv3_a3Zn)
          }
      }
end Rec }

Relevant STG code.

let {
  sat_s5ZW [Occ=Once] :: GHC.Types.Double -> GHC.Types.Double
  [LclId] =
      \r [y_s5ZQ]
          case ipv_s5ZJ of {
            GHC.Types.D# ww3_s5ZS [Occ=Once] ->
                let {
                  sat_s5ZU [Occ=Once]
                    :: GHC.Types.Double -> GHC.Types.Double
                  [LclId] =
                      \r [x_s5ZT]
                          GHC.Float.timesDouble x_s5ZT y_s5ZQ;
                } in 
                  case
                      $wintegrate1D_r5YG 0.0## ww3_s5ZS sat_s5ZU
                  of
                  ww4_s5ZV [Occ=Once]
                  { __DEFAULT -> GHC.Types.D# [ww4_s5ZV];
                  };
          };
} in ...