[Caml-list] "Inconsistent assumptions" when moving files across subdirectories

[Josh Watzman <jwatzman@fb.com>]

I've noticed that it's pretty easy to confuse ocamlc/ocamlopt when moving a module across subdirectories. Here's an example, the most minimized repro I could get; it uses ocamlbuild, but a similar problem happens if you use OCamlMakefile and I assume other build systems. https://gist.github.com/jwatzman/9979951afb5b87304c18 -- running that will consistently terminate with the dreaded

> Error: Files main.cmx and a/quux.cmx
>        make inconsistent assumptions over interface Quux

(The script flips the quux module back and forth twice, but that's only to exhibit the problem on both 4.01 and 4.02; you can get the same problem with only one move of quux.ml, but which way you need to move it depends on which version of ocaml you're using.)

A clean build will of course resolve the problem, but that's quite annoying to have to go broadcast to a large team, particularly when the build may take many minutes, and when this problem is specific to the OCaml parts of our system (a humongous C++ codebase never requires a clean rebuild). Renaming a module across subdirectories doesn't seem like that uncommon of an operation.

The root problem seems to be that ocamlc/ocamlopt are picking up build artifacts by directory only, and can't be explicitly told which artifacts to pick up, and so they are picking up the "wrong" quux.cmi/cmx left over in a build directory, which ocamlbuild should be cleaning up. Is that right? Is there any way to tell ocamlc/ocamlopt not to do things by directory, but to be more explicit, for the usages of build systems?

Not working around this limitation of ocamlc/ocamlopt seems like a bug in ocamlbuild, no? I'm a bit surprised by it though, given that I've found the same problem in other build systems -- have other folks not run into this? How do other teams deal with this, trying to avoid clean builds?

Thanks!
Josh Watzman