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It seems like a hidden non-generalized type variable remains in some inferred signatures, which leads to strange errors #7601

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vicuna opened this Issue Jul 31, 2017 · 7 comments

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vicuna commented Jul 31, 2017

Original bug ID: 7601
Reporter: mandrykin
Assigned to: @garrigue
Status: resolved (set by @garrigue on 2017-09-12T23:35:43Z)
Resolution: fixed
Priority: low
Severity: minor
Platform: x86_64
OS: Linux 4.4.0
OS Version: Ubuntu 16.04.2
Version: 4.05.0
Fixed in version: 4.06.0 +dev/beta1/beta2/rc1
Category: typing

Bug description

The following fragment:

module type Analysis = sig
type t
type 'a maybe_region =
[< Location of t | Value of t
| None ] as 'a val of_var : ?f:string -> string -> [ Location of _ | Value of _ | None ] maybe_region
end

module Make (Analysis : Analysis) = struct
include Analysis
let of_var = of_var ~f:""
end

produces the following inclusion error:

At position module Make(Analysis) : Values do not match:
val of_var : string -> [ Location of t | None | Value of t ] is not included in val of_var : string -> [ Location of t | None | Value of t ]

(no observable difference in types and no visible type variables) (in 4.05.0)

However, if either generalization is requested for the value `of_var' using eta-expansion or a type annotation that does not involve the type maybe_region is added, the error disappears. So either of the following works:

module Make (Analysis : Analysis) = struct
include Analysis
let of_var x = of_var ~f:"" x
end

or

module Make (Analysis : Analysis) = struct
include Analysis
let of_var : _ -> [ Location of _ | None | `Value of _] = of_var ~f:""
end

but not

module Make (Analysis : Analysis) = struct
include Analysis
let of_var : _ -> [ Location of _ | None | `Value of _] maybe_region = of_var ~f:""
end

Additional information

The types like [A of _ | B of _ | C] t where 'a t = 'a constraint 'a = [A of t1 | B of t2 | C of t3 | D of ... | .... | X of tn] are convenient for avoiding repeating the types of the polymorphic constructors if they are taken from some pre-defined set e.g. using
[Location of _ | None] maybe_region instead of
[Location of U.t * [None | Value of U.t] | None].

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vicuna commented Jul 31, 2017

Comment author: mandrykin

Added another more minimal example, the presence of an abstract type among the types of constructors does also matter.

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vicuna commented Aug 2, 2017

Comment author: @garrigue

Interesting. This bug is related to the change in 4.04, where locally defined abstract types cannot be used to instantiate non-generalizable type variables once the defining module is left.

First remark: there is an easy workaround, adding a variance annotation:

type +'a maybe_region = ...

I'm thinking of a fix that wouldn't break principality for this case.

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vicuna commented Aug 2, 2017

Comment author: @garrigue

Tentative fix: #1272

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vicuna commented Aug 29, 2017

Comment author: @garrigue

Fixed in 4.05 branch after 4.05.0 by #1272.

Note that the fix requires copying signatures, which seems to increase the size of cmi's when there is no mli.

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vicuna commented Sep 6, 2017

Comment author: @garrigue

Fix was breaking camomille and Frama-C. Reverted it.

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vicuna commented Sep 6, 2017

Comment author: @garrigue

New tentative fix: #1320

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vicuna commented Sep 12, 2017

Comment author: @garrigue

Fixed in trunk by commit ab7af2a, by replacing row variables of static variant types with Tnil.

@vicuna vicuna closed this Sep 12, 2017

@vicuna vicuna added the typing label Mar 14, 2019

@vicuna vicuna added the bug label Mar 20, 2019

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