Looks like there is a multi-version incompatibility:

• 4.08 to 4.11 don't have Compenv.Exit_with_status
• 4.12 to 4.13 don't throw errors on invalid #use
• 4.14 works

It's quite hard to build a compatibility wrapper in Compat_top to work around this if there isn't a consistent way to get the error between different versions, especially since before < 4.14 these don't seem to trigger errors.

I looked a bit around in the source code of 4.12 vs 4.14 and it seems like execute_phrase has seen some considerable rework between the releases which has probably lead to better error reporting.

I played around a bit in #384 to get it to compile on older versions, which is not too difficult. Two issues remain:

1. The output disappears. This seems to be because in <4.14 #use is hardcoded to write to std_out whereas in 4.14 it will write to stdout or stderr. I think it should be possible to patch #use to use stderr on older versions too, if we just redefine the relevant directives.
2. No Compenv.Exit_with_status exception is thrown. I have yet to figure out how this happens in 4.14, because even despite the directive failing and printing to the void, older versions happily continue on, even returning true in the evaluation because directives always evaluate to true.

Okay, I looked into this and I think I resolved all the issues here:

1. Redefined a lot of directives (those exposed in Topdirs) to write to stderr instead of stdout, so they get captured. 4.14 does that conditionally, but here we expect it to be always to be on stderr. There's some subtlety to this because depending on the supported version this looks different. In particular, #load can only be overridden in 4.13+ because older versions don't expose load_file which does not recurse. We could try to copy-paste the definition but I opted to avoid copying too much code and keep the complexity (somewhat) low at the cost of less than 100% identical behavior.
2. The redefined directives no longer use ignore to suppress the error but throw Exit_with_status, similar to how it is done on 4.14, but with our own exception so it works on older OCaml versions as well.

The directives are all implemented as custom mdx_<original-name> directives, because when I tried to override the existing ones there is a race condition between the original and the patched ones. So instead the code rewrites e.g. #use into #mdx_use on OCaml <4.14 which avoids the headache altogether.

This was a fun challenge, teaching me a thing or two how the directives in the toplevel work.

Thanks - I hadn't realised how much work it would be to support older versions!

I could move stuff out of Compat_top but besides being "somewhere else" I feel there is not particularly much that can be done, given this is all icky reimplementing of internal libs, trying to duplicate the least amount of code.

Thanks for finding the issues with the constraints, looks like I copypasted exactly the one line that had a problem.