decodeDouble_Int64 rewrite rule is incorrect (core lint error - compiling AD)

module Double
  (
  eexponent
  ) where

-- spine lazy, value strict list of doubles
data List
  = Nil
  | {-# UNPACK #-} !Double :! List

infixr 5 :!

newtype TowerDouble = Tower { getTower :: List }

primal :: TowerDouble -> Double
primal (Tower (x:!_)) = x
primal _ = 0

eexponent :: TowerDouble -> Int
eexponent = exponent . primal

 > ~/head.hackage/ghc/bin/ghc Double.hs -O -dcore-lint

*** Core Lint errors : in result of Simplifier ***
Double.hs:20:1: warning:
    Argument value doesn't match argument type:
    Expected arg type: Int64#
    Actual arg type: Int#
    Arg: 0#
    In the RHS of eexponent :: TowerDouble -> Int
    In the body of lambda with binder x_aSF :: TowerDouble
    In the body of letrec with binders $j_sTw :: Double# -> Int
    In a case alternative: (Nil)
    Substitution: <InScope = {}
                   IdSubst   = []
                   TvSubst   = []
                   CvSubst   = []>

Which we note is because of this incorrect rule firing:

Rule fired                                                                      
    Rule: decodeDouble_Int64#                                                   
    Module: (BUILTIN)                                                           
    Before: GHC.Prim.decodeDouble_Int64# ValArg 0.0##                           
    After:  (# 0#, 0# #)                                                        
    Cont:   Select nodup ds_aTq                                                 
            Stop[BoringCtxt] GHC.Types.Int  

but the first part of the tuple should be an Int64.

primop   DoubleDecode_Int64Op   "decodeDouble_Int64#" GenPrimOp
   Double# -> (# Int64#, Int# #)
   {Decode 'Double#' into mantissa and base-2 exponent.}
   with out_of_line = True