• They are no longer included in the upstream build, since they will use extensions that the upstream compiler doesn't support. Hence they do not need a Makefile-based build.
• The tests for exported modules have been moved from ocaml/testsuite/ to testsuite/, so will only run in flambda-backend builds. The upstream compatible versions (only modules.ml and upstream_compatible.ml) are still under ocaml/testsuite.

As far as I know, the makefile-based build inside this repository is used not by the upstream compiler, but by our compiler. ocaml/testsuite/ also contains numerous tests that only work inside our compiler, such as layouts tests. So I'm confused why would we move the otherlibs tests to testsuite/.

Some time ago we switched flambda-backend builds to use dune whenever available - I confirmed the lib-extensions tests still run under make test here, so it seems fine.

The layouts tests actually work in the upstream compiler build because they only require frontend features present in ocaml/. As of this PR it's not yet necessary to move the otherlibs tests, but it will be when I add the Float32 module, because it requires backend features that are not present in ocaml/.

I think you should move upstream_compatible to be flambda-only then too. upstream_compatible does not actually mean "builds as is with the upstream compiler", instead it's "something we provide an upstream-compatible substitute shim for, even if it degrades performance". So, for example, Float_u can be flambda-only, yet still be in upstream_compatible if we have a module with the same interface (modulo extension erasure stuff) and an upstream-compatible implementation, even if that implementation just uses normal float.

Sounds good, upstream_compatible is now treated the same as the others

Should we merge this?