I once wondered about subtyping[1] too. Looks like there's a possible solution - MLsub[2][3]. But it will require significant amount of work to modify OCaml inference engine to work that way. For now I just use functors for types with the shared fields. It doesn't add too much lines of code either.

[1] https://discuss.ocaml.org/t/function-argument-polymorphism/3...

[2] https://www.cl.cam.ac.uk/~sd601/papers/mlsub-preprint.pdf

[3] https://github.com/stedolan/mlsub