point where ghc remove unuseful function

[Luca Torella]

Hi all.

I'm trying to find the point where GHC removes the function that  
couldn't be executed. For example if I declare a function like:

f x = 3
f x = 5

I know that f x = 5 can0t be executed ever. The desugarer removes it,  
just look the output after the desugaring phase:

Rec {
Main.f :: forall t_aOL t_aOM. (GHC.Num.Num t_aOM) => t_aOL -> t_aOM
[Exported]
[]
Main.f =
   \ (@ t_aOL) (@ t_aOM) ($dNum_aOT :: {GHC.Num.Num t_aOM}) ->
     __letrec {
       f_aOQ :: t_aOL -> t_aOM
       []
       f_aOQ = \ (x_aOF :: t_aOL) -> lit_aON;
       fromInteger_aP3 :: GHC.Num.Integer -> t_aOM
       []
       fromInteger_aP3 = fromInteger_aOR;
       lit_aON :: t_aOM
       []
       lit_aON = fromInteger_aP3 (GHC.Num.S# 3);
       fromInteger_aOR :: GHC.Num.Integer -> t_aOM
       []
       fromInteger_aOR = GHC.Num.fromInteger @ t_aOM $dNum_aOT;
       lit_aOP :: t_aOM
       []
       lit_aOP = fromInteger_aOR (GHC.Num.S# 5);
     } in  f_aOQ
end Rec }

after the desugarer the second function has been removed as you can see.

But where is the actual point where it does this operation? I thought  
was some function in the Check.lhs file, but putting a lot of trace I  
discovered this isn't the point. Someone could help me, pointing the  
right function which does it?


thanks in advance,
Luca.