I'm using a syntax extension that installs to ocamlc -where/dir.
I've tried to use it with
$ camlp4 ... -I +dir pa_someext.cmo ...
but with no success:
Camlp4: Uncaught exception: DynLoader.Error ("pa_someext.cmo", "file not found in path")
However, ocamlc/ocamlopt does handle this correctly:
$ ocamlc -I +dir pa_someext.cmo -o somebin
shows to me that pa_someext.cmo is found and processed.
Looking at camlp4 sources I've figured out why camlp4 doesn't handle +directory notation: function "include_dir" in "$ocamlsources/camlp4/Camlp4/Struct/DynLoader.ml" doesn't have any special processing of "+directory" syntax.
IMHO every ocaml-related tool (toplevel, compiler, preprocessor) should have the same semantics for -I option.