Comment author: @garrigue

In order to handle properly optional arguments, application is n-ary.
That is [(f 1 2) ~foo:()] andf [f 1 2 ~foo:()] are not equivalent.
It looks like camlp4 is merging the two applications into one.

Jacques

Comment author: cfalls

There's another issue that I think is just another manifestation of this one. In Core, we define:

let failwithf fmt = ksprintf (fun s () -> failwith s) fmt

Then, [(failwithf "foo" ()) 3] produces a warning about unused arguments only when not using camlp4.

(Note that [failwithf "foo" () 3] doesn't produce the warning in either case. It would be nice if it did, but that's a separate issue.)

Comment author: @bobzhang

garrigue is right. camlp4 seems to merge the two applications

Comment author: @damiendoligez

Transferred to camlp4/camlp4#70