X := X; Compiler Warning?

[Warren <ve3wwg@gmail.com>]

This weekend, I discovered a problem after some
effort in debugging. The culprit was something
stupid like this:

  Position_X : Line_Index := ...
begin

  loop
    ..
    Position_X := Position_X;  -- whoops
    ..
    exit when ...;
  end loop;

Obviously, that should have been coded as:

    Position_X := Position_X + 1;

As outlined here, it is trivial to spot. 

But with the longer variable names (in the original
code), I somehow missed it in amongst all of the
other statements.

Now isn't this something that is normally reported
as a warning?  I'm using gnat, so perhaps I need an
additional compiler option.

Thanks, Warren