Type equality broken for conjunctive polymorphic variant tags

[vicuna]

Original bug ID: 5927
Reporter: @lpw25
Assigned to: @garrigue
Status: resolved (set by @garrigue on 2017-03-14T10:20:51Z)
Resolution: fixed
Priority: normal
Severity: minor
Target version: undecided
Fixed in version: 4.06.0 +dev/beta1/beta2/rc1
Category: typing
Tags: patch
Monitored by: @lpw25

Bug description

I think that the following behaviour is a bug:

type 'a foo = 'a constraint 'a = [< `Tag of & int];;

Error: Type declarations do not match:
type 'a foo = 'a constraint 'a = [< Tag of & int ] is not included in type 'a foo = 'a constraint 'a = [< Tag of & int ]

I've attached a patch that seems to fix it.

File attachments

• fix-amp-equal.diff
• fix-amp-equal2.diff

[vicuna]

Comment author: @lpw25

I've attached a new fix that is probably more consistent with other behaviour.

[vicuna]

Comment author: @garrigue

Actually, the failure in this case is intentional, as this conjunction is clearly impossible.
On the other hand, it could be accepted in a more general way: & t1 is probably equivalent to & t2 for any t1 and t2...
(Oops, there is also the problem of free variables)

[vicuna]

Comment author: @lpw25

Even if the conjunction is impossible, the type itself is valid and should at least be considered equivalent to itself.

Also, the current error is unclear and only occurs during module inclusion checks. For example:

module type S = sig type 'a foo constraint 'a = [< `Tag of & int ] end;;

Error: Module type declarations do not match:
module type S =
sig type 'a foo constraint 'a = [< Tag of & int ] end does not match module type S = sig type 'a foo constraint 'a = [< Tag of & int ] end
At position module type S =
Modules do not match:
sig type 'a foo constraint 'a = [< Tag of & int ] end is not included in sig type 'a foo constraint 'a = [< Tag of & int ] end
At position module type S =
Type declarations do not match:
type 'a foo constraint 'a = [< Tag of & int ] is not included in type 'a foo constraint 'a = [< Tag of & int ]
Their constraints differ.

[vicuna]

Comment author: @garrigue

Fixed by commit c21ffce, in a slightly more general way.