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Relaxed value restriction broken with principal #7285

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vicuna opened this Issue Jul 6, 2016 · 4 comments

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vicuna commented Jul 6, 2016

Original bug ID: 7285
Reporter: @lpw25
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2017-09-24T15:33:06Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.03.0
Fixed in version: 4.04.0 +dev / +beta1 / +beta2
Category: typing
Tags: testsuite, typing
Monitored by: @bobzhang @gasche @hcarty

Bug description

In principal mode generalize_structure is used for contravariant variables. However, generalize_structure ignores variables which have already been generalized. This means that instead of generalizing variables which occur only in covariant positions, we generalize any variable which is first encountered in a covariant position.

For example, the following code is allowed and results in a segmentation fault:

      OCaml version 4.03.0

#principal true;;

module M : sig

  type (+'a, -'b) foo
  val apply : ('a, 'b) foo -> 'b -> 'a
  val f : unit -> ('a, 'a) foo
end = struct
  type ('a, 'b) foo = 'b -> 'a
  let apply f x = f x
  let f () =
    let r = ref None in
      fun x ->
        match !r with
        | None -> r := Some x; x
        | Some y -> r := Some x; y
end;;

module M :
sig
type (+'a, -'b) foo
val apply : ('a, 'b) foo -> 'b -> 'a
val f : unit -> ('a, 'a) foo
end

let f = M.f ();;

val f : ('a, 'a) M.foo =

M.apply f 0;;

  • : int = 0

print_endline (M.apply f "World");;

Process ocaml-toplevel segmentation fault (core dumped)

Reversing the order of the type arguments of M.foo gives a type error as expected, since the variable is encountered in a contravariant position first.

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vicuna commented Jul 6, 2016

Comment author: @gasche

A rather bad bug indeed. (Unlike some other tricky type-system bugs,) I think we could have caught this one with a slightly more proactive testing strategy, testing as much generalization behaviors as possible. Marking the issue as testsuite-related in case people are interested in working on this.

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vicuna commented Jul 7, 2016

Comment author: @garrigue

Fixed in trunk by commit 9d77b26.

Part of the reason it went undetected is that -principal was not used much until recently.
And I admit that the change to generalize_contravariant was stupid (it should have come with a few tests...)

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vicuna commented Jul 7, 2016

Comment author: @alainfrisch

it went undetected is that -principal was not used much until recently.

Has something changed recently which makes -principal easier to use, or is it an empirical observation?

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vicuna commented Jul 7, 2016

Comment author: @garrigue

-principal didn't change, but it is now used for the compiler itself, giving it more visibility.

@vicuna vicuna closed this Sep 24, 2017

@vicuna vicuna added the typing label Mar 14, 2019

@vicuna vicuna added the bug label Mar 20, 2019

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