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Module alias is not taken into account when checking module type compatibility (in a class type) #7036

vicuna opened this issue Nov 2, 2015 · 1 comment


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commented Nov 2, 2015

Original bug ID: 7036
Reporter: mandrykin
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2017-02-16T14:14:37Z)
Resolution: fixed
Priority: low
Severity: minor
Platform: x86_64
OS: Linux 3.19.0
OS Version: Ubuntu 15.04
Version: 4.02.3
Fixed in version: 4.03.0+dev / +beta1
Category: typing
Monitored by: @hcarty

Bug description

The following reduced example:

module M = struct
module type S = sig type a val v : a end
type 'a s = (module S with type a = 'a)

module B = struct
class type a = object method a : 'a. 'a M.s -> 'a end

module M' = M
module B' = B

class b : B.a = object
method a : 'a. 'a M.s -> 'a = fun (type a) ((module X) : (module M.S with type a = a)) -> X.v

class b' : B.a = object
method a : 'a. 'a M'.s -> 'a = fun (type a) ((module X) : (module M'.S with type a = a)) -> X.v

is not accepted in both 4.02.3 and 4.03.0+trunk (4.03.0+dev11-2015-10-19):

Error: The class type object method a : 'a M'.s -> 'a end
is not matched by the class type B.a
The class type object method a : 'a M'.s -> 'a end
is not matched by the class type object method a : 'a M.s -> 'a end
The method a has type 'a. 'a M'.s -> 'a but is expected to have type
'a. 'a M.s -> 'a
Type 'a M'.s = (module M'.S with type a = 'a)
is not compatible with type 'a0 M.s = (module M.S with type a = 'a0)

although as module M' is an alias for module M, module types M.S and M'.S actually refer to the same module type.

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commented Nov 5, 2015

Comment author: @garrigue

Fixed in trunk in commit f32db11.

Structural equality on package types only work if the types do not contain free type variables (both Tvar and Tunivar). So we still need to expand the module path for aliases.

@vicuna vicuna closed this Feb 16, 2017

@vicuna vicuna added the typing label Mar 14, 2019

@vicuna vicuna added the bug label Mar 20, 2019

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