Original bug ID: 7055
Reporter: @sliquister
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2017-02-16T14:16:27Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.02.3
Fixed in version: 4.03.0+dev / +beta1
Category: typing

Bug description

The following program:

module M : sig
type +'rw t constraint 'rw = [> A ] end = struct type 'rw t = unit constraint 'rw = [> A ]
end

triggers a type error:

File "/tmp/a.ml", line 3, characters 6-61:
Error: Signature mismatch:
Modules do not match:
sig type 'a t = unit constraint 'a = [> A ] end is not included in sig type +'a t constraint 'a = [> A ] end
Type declarations do not match:
type 'a t = unit constraint 'a = [> A ] is not included in type +'a t constraint 'a = [> A ]
File "/tmp/a.ml", line 2, characters 2-38: Expected declaration
File "/tmp/a.ml", line 4, characters 2-44: Actual declaration
Their variances do not agree.

Notice that the t in the structure doesn't have a variance
annotation. If I put the variance annotation on the structure too, the
code types. If I remove the constraint, the code types.

I don't know if that's a known limitation or a bug. Of course it's not
a big problem, although the error in my original program was rather
confusing. It looked like a more complicated version of this:

module M : sig
type +'rw t constraint 'rw = [> A ] type +'rw t2 = 'rw t end = struct type +'rw t = unit constraint 'rw = [> A ]
type 'rw t2 = 'rw t
end

I had the same error as above but on t2, despite t2 being obviously covariant
in 'rw in both the interface and the implementation.