MPR#4943: ocaml_lib is not specific enough with tags, leading to highly confusing error message

[damiendoligez]

PR transferred from https://caml.inria.fr/mantis/view.php?id=4943
[original reporter: @ygrek]

Consider this ocaml_lib code (in ocaml_utils.ml) :

    if byte then
      flag_and_dep ["ocaml"; tag_name; "link"; "byte"] (libpath^".cma");
    if native then
      flag_and_dep ["ocaml"; tag_name; "link"; "native"] (libpath^".cmxa");

When the final linking target is a program - everything is ok. If it is a library - ocamlc will work, but ocamlopt will refuse to link cmxa into another cmxa, producing highly confusing error message : ...cmxa is not a compilation unit description. Of course, this situation can be avoided by carefully specifying tags like true and not <*.cmxa>: use_lib but this quickly becomes cumbersome in non-trivial projects. So the suggestion is for ocaml_lib to be more specific with tags :

    if byte then
      flag_and_dep ["ocaml"; tag_name; "link"; "program"; "byte"] (libpath^".cma");
    if native then
      flag_and_dep ["ocaml"; tag_name; "link"; "program"; "native"] (libpath^".cmxa");

Are those the only link targets : library and program ?

---

$ cat _tags 
true: use_str

$ cat q.ml
let x = Str.regexp "a"

$ cat q.mllib
Q

$ ocamlbuild q.cma
Finished, 3 targets (0 cached) in 00:00:00.

$ ocamlbuild q.cmxa
+ /usr/bin/ocamlopt.opt -a str.cmxa q.cmx -o q.cmxa
File "_none_", line 1, characters 0-1:
Error: /usr/lib/ocaml/str.cmxa
is not a compilation unit description.
Command exited with code 2.