match case unused

 I have to rewrite a not function with pattern matching.
 
So I did this : 
 
let not x = 
   match x with 
     x -> false
     | _ -> true ;;
 
but now I see this warning : Warning 11: this match case is unused.
 
Which is correct because on output both not true and not false gives the output true.
Which I find wierd.
 
not true 
match true with 
  true -> false 
 
So the expected output is false 
 
Can someone explain to me where my thinking took the wrong turn ?
 
Roelof