I'm sorry, I noted this change here but failed to make sure that it was seen by anyone working on dune.

No worries and thanks @kit-ty-kate for the patch. Nice to know about this OCaml PR BTW, seems like we might finally be able to get rid of ppx_pipebang at Jane Street.

BTW, I'm curious how the OCaml PR relates to this change. I'm guessing this is because of the |> in let output = ... in, but I don't see the link to the t variable.

@jeremiedimino there was some discussion of the need to add this annotation in the ocaml PR comments, see e.g. ocaml/ocaml#10081 (comment) . TL;DR: it seems that this code was relying on (non-principal) horizontal type propagation.

i forgot to specify, this probably needs a cherry-pick in main as well

@jberdine thanks for the pointer. Without digging too much into this, my initial interpretation is: "where type annotations are needed depend on implementation details in the compiler". Am I far from the truth? :)

@kit-ty-kate, cherry-picked in main.

@jberdine thanks for the pointer. Without digging too much into this, my initial interpretation is: "where type annotations are needed depend on implementation details in the compiler". Am I far from the truth? :)

Yeah, that's right. Except that if you compile with -principal it is AFAIU deemed a bug in the compiler for such details to matter.