Original bug ID: 7699
Reporter: ubsan
Status: resolved (set by @xavierleroy on 2018-01-12T13:01:08Z)
Resolution: not fixable
Priority: normal
Severity: minor
Version: 4.06.0
Category: typing

Bug description

Given two recursive modules T1 and T2, and an opaque type T1.t which is equal to b, references to T1.t in T2 are not normalized to have type b in T1.

Steps to reproduce

module rec Foo : sig
  val make: Bar.t
end = struct
  let make = Bar.make
end and Bar : sig
  type t
  val make: t
end = struct
  type t = unit
  let make = ()
  let _: unit = Foo.make
end

gives the error:

File "./test.ml", line 11, characters 16-24:
Error: This expression has type Bar.t but an expression was expected of type
unit

Additional information

One can get around this issue by not using type aliases - I've gotten around this in my own code with a single-variant variant type.