Comment author: @lpw25

This comes from the potential ambiguity with:

module rec X : T with type t = int
                   and type t = int

that is resolved in favour of shifting the and.

It can be avoided using parentheses:

module rec X : (T with type t = int)
       and Y : T

I think that avoiding the conflict isn't really possible in an LR(1) grammar because you'd need to know whether to shift or reduce the and without looking at the Y. Maybe you could extensively re-engineer the grammar so that the conflict was avoided, but I suspect its quite difficult to do so.

Comment author: @yallop

Ah, this is already noted in the manual!

Note that, in the specification case, the module-types must be parenthesized if they use the with mod-constraint construct.

It could be made a bit more precise, I suppose, by mentioning that the parentheses aren't needed on the last member in the group, but it hardly seems worth it.