Comment author: @alainfrisch

Another case of incompleteness of the warning is related to local open (even without the new explicit local open construction):

module A = struct let x = 1 end
let f () = let x = 2 in let module M = struct open A let a = x + 1 end in ()

Here the local "let x = ..." is not used. To see this, the check should be implemented in the type-checker, not as a purely syntactic pass. Doing the detection in the type-checker opens the door to more powerful detection of useless declaration (e.g. module components [types, values] which are not used internally and are hidden by the signature). We use such a patch at LexiFi, and it helped us find a lot of dead code and a few bugs.

Another example with classes:

class c = object val x = 1 end
let f () = let x = 2 in object inherit o method y = x end

or even, simply:

let f () = let x = 2 in object val x = 1 method y = x end

(this last example could probably be fixed even with the current syntactic approach).