Comment author: @Octachron

As far as I can see, this is a local substitution, a new feature of 4.08? Your code example results in a syntax error with 4.07. Nevertheless, the error message might fall in an unfortunate local minimum: rewriting the local substitution to

type t := A.t = A of A.t

yields:

5 | type t := A.t = A of A.t
^^^^^^^^^^^^^^^^^^^^^^^^
Error: Only type synonyms are allowed on the right of :=

Comment author: @nojb

Indeed, you are right, I got confused -- sorry for the noise!

Comment author: @gasche

octachron, is the behavior you spotted a bug? The message you have (for the intuitively "correct" definition) looks like a parsing-error message delayed to type-checking time (I'm fine with this, but we may move it back to the parser one day if it has proper error messages), but the error of nojebar suggests that type-checking happens before the construction is rejected. Does it mean that the not-allowed rejection happens too late in the pipeline?

Comment author: @Octachron

It does look like the rejection happens too late.

Comment author: @trefis

https://github.com/ocaml/ocaml/blob/trunk/typing/typemod.ml#L1171

Feel free to move the code checking the shape of the declarations before the actual type-checking part.
But I agree that the error should come from the parser, when we finally get errors from there.

Comment author: @Octachron

The error message should be improved with #2230 . So thanks for the report nojebar!