This appears to be fixed in trunk:

$ TERM=dumb make runtop

        OCaml version 4.11.0+dev0-2019-10-18

# type 'a t = private < x : int; .. > as 'a;;
Line 1:
Error: Type declarations do not match:
         type 'a t = private < x : int; .. > constraint 'a = 'a t
       is not included in
         type 'a t
       Their constraints differ.

This issue has been open one year with no activity. Consequently, it is being marked with the "stale" label. What this means is that the issue will be automatically closed in 30 days unless more comments are added or the "stale" label is removed. Comments that provide new information on the issue are especially welcome: is it still reproducible? did it appear in other contexts? how critical is it? etc.

I don't know what the issue was and whether it really was fixed. Even on 4.07, calling ocaml /tmp/foo.ml would print correct locations for this sentence. Interactive use with TERM=dumb does indeed show nonsensical locations, and later versions of the toplevel stop doing this, but I don't know if there ever was a nonsensical location parsed for the sentence (maybe the dummy-loc came from some other place) and if this location actually changed, or we just got better at not printing the location when we don't know.

In any case, indeed the user-facing issue (-1 in a location) is gone in 4.08 and later (note: 4.08 still prints the file as _none_ which is a bit confusing, but then 4.09 and later don't do this either), so I will close the issue as not-an-issue-anymore.

Thanks for the reporting work.