RULE left-hand side too complicated to desugar

GHC reports multiple "RULE left-hand side too complicated to desugar" warnings for the fast-math package.

Here is an testcase, extracted from fast-math/Numeric/FastMath/Approximation.hs:

{-# LANGUAGE MagicHash #-}
module T10555 where

import GHC.Exts

{-# RULES

"double /,+ distribute" forall x y1 y2. (y1 /## x) +## (y2 /## x) 
    = (y1 +## y2) /## x

"double /,- distribute" forall x y1 y2. (y1 /## x) -## (y2 /## x) 
    = (y1 -## y2) /## x

"float /,- distribute" forall x y1 y2. (y1 `divideFloat#` x) `minusFloat#` (y2 `divideFloat#` x) 
    = (y1 `minusFloat#` y2) `divideFloat#` x

  #-}

2$ ghc-7.10.2 T10555.hs  -fforce-recomp
[1 of 1] Compiling T10555           ( T10555.hs, T10555.o )

T10555.hs:8:1: Warning:
    RULE left-hand side too complicated to desugar
      Optimised lhs: case /## y2 x of wild_00 { __DEFAULT ->
                     (case /## y1 x of wild_X2 { __DEFAULT -> +## wild_X2 }) wild_00
                     }
      Orig lhs: case /## y2 x of wild_00 { __DEFAULT ->
                (case /## y1 x of wild_00 { __DEFAULT -> +## wild_00 }) wild_00
                }

T10555.hs:11:1: Warning:
    RULE left-hand side too complicated to desugar
      Optimised lhs: case /## y2 x of wild_00 { __DEFAULT ->
                     (case /## y1 x of wild_X2 { __DEFAULT -> -## wild_X2 }) wild_00
                     }
      Orig lhs: case /## y2 x of wild_00 { __DEFAULT ->
                (case /## y1 x of wild_00 { __DEFAULT -> -## wild_00 }) wild_00
                }

T10555.hs:14:1: Warning:
    RULE left-hand side too complicated to desugar
      Optimised lhs: case divideFloat# y2 x of wild_00 { __DEFAULT ->
                     (case divideFloat# y1 x of wild_X2 { __DEFAULT ->
                      minusFloat# wild_X2
                      })
                       wild_00
                     }
      Orig lhs: case divideFloat# y2 x of wild_00 { __DEFAULT ->
                (case divideFloat# y1 x of wild_00 { __DEFAULT ->
                 minusFloat# wild_00
                 })
                  wild_00
                }