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OCaml compiler loops on an example using GADTs #6174

vicuna opened this issue Sep 12, 2013 · 2 comments


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commented Sep 12, 2013

Original bug ID: 6174
Reporter: ckeller
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2015-12-11T18:24:17Z)
Resolution: fixed
Priority: normal
Severity: major
Version: 4.00.1
Fixed in version: 4.01.1+dev
Category: ~DO NOT USE (was: OCaml general)
Monitored by: @gasche

Bug description

The OCaml compiler seems to loop on the following program (which I also attached):

type _ t = C : ((('a -> 'o) -> 'o) -> ('b -> 'o) -> 'o) t
let f : type a o. ((a -> o) -> o) t -> (a -> o) -> o =
fun C k -> k (fun x -> x)

Additional information

It loops as well with the version 4.02.0dev+trunk installed with opam.

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commented Oct 12, 2013

Comment author: hnrgrgr

After some debugging, the loop appears in the 'lower_args' function while type-checking the application (in Typecore.type_expect_, case Pexp_apply).

There is the following equalities "o = ex#2 -> ex#1" where "ex#1 = ex#2 -> ex#1". Each expansion of ex#1 allocates an new 'type_expr', and the 'seen' list is infinitely growing.

Still, I do not understand why the following generalisation is accepted:

type _ t = C : ((('a -> 'o) -> 'o) -> ('b -> 'o) -> 'o) t
let f = fun C -> ()
let f1 : type a o. ((a -> o) -> o) t -> unit = f (* refused )
let f2 : type a o. ((a -> o) -> o) t -> unit = fun C -> () (
accepted *)


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commented Oct 28, 2013

Comment author: @garrigue

Fixed in 4.01 and trunk, revisions 14246-14248.
In 4.01 only the non-rectypes case is fixed (was due to a typo).
Trunk fully fixes the problem, by ensuring that the abbreviation cache is not cleared in the middle of a potentially recursive traversal (some other cases may still be there).

@vicuna vicuna closed this Dec 11, 2015

@vicuna vicuna added the bug label Mar 20, 2019

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