0004775: compiler crashes on crazy types - MantisBT

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ID

Project

Category

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Date Submitted

Last Update

0004775

OCaml

OCaml general

public

2009-04-23 23:54

2011-05-29 12:14

Reporter

cfalls

Assigned To

garrigue

Priority

normal

Severity

crash

Reproducibility

always

Status

closed

Resolution

fixed

Platform

OS

OS Version

Product Version

3.11.0

Target Version

Fixed in Version

3.11.1+dev

Summary

0004775: compiler crashes on crazy types

Description

The following program crashes several versions of the compiler. I discovered this bug when fiddling around with type constraints -- I doubt it would be triggered by any program that was written to do something useful.

=============== ~/bug2.ml ====================
module type Poly = sig
  type 'a t = 'a constraint 'a = [> ]
end

module Combine (A : Poly) (B : Poly) = struct
  type ('a, 'b) t = 'a A.t constraint 'a = 'b B.t
end

module C = Combine
  (struct type 'a t = 'a constraint 'a = [> ] end)
  (struct type 'a t = 'a constraint 'a = [> ] end)
=============== ~/bug2.ml ====================

> /janelibs/ocaml-3.11.0+jane3_64/bin/ocaml ~/bug2.ml
Fatal error: exception Ctype.Unify(_, _)
(Program not linked with -g, cannot print stack backtrace)

> /janelibs/ocaml-3.11.0+jane3_64/bin/ocaml -version
The Objective Caml toplevel, version 3.11.0+jane3

> /usr/local/home/godi310/godi/bin/ocaml ~/bug2.ml
Fatal error: exception Ctype.Unify(_, _)
(Program not linked with -g, cannot print stack backtrace)

> /usr/local/home/godi310/godi/bin/ocaml -version
The Objective Caml toplevel, version 3.10.1+dev0 (2007-05-21)

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Notes
(0004924) garrigue (manager) 2009-04-27 10:41	Thanks for this interesting report. Indeed your crazy types are not crazy at all, and I believe the code you describe should be accepted. But the cause of the bug, in Ctype.nondep_type_rec, is the use of unification in expand_abbrev combined with a special representation with Tsubst nodes, which was introduced about 10 years ago, so I believe that no version since 2.99 can compile it. It would be interesting to see whether ocaml 2.04 can compile it, as I have my doubt about the correctness of the previous code too. I'm still looking for a workaround...

(0004956) garrigue (manager) 2009-05-19 07:44	Actually the first explanation was wrong. The real trouble here is the use of a curried functor. If you uncurry it (use two submodules as argument), then this program is accepted. A real fix would be either in Mtype or Typemod. As a temporary fix, I catch Unify in Ctype.nondep_type_rec. Curiously, this does not results in a failure, but just in making t abstract in the result.

(0004958) garrigue (manager) 2009-05-19 10:25	A better solution is now commited in head branch (3.12+dev), solving both the problem with nondep_type (do not modify types during copying) and Mtype.nondep_mty (extend environment when eliminating in functors). Your example is now typable with this version.

Issue History
Date Modified	Username	Field	Change
2009-04-23 23:54	cfalls	New Issue
2009-04-27 10:41	garrigue	Note Added: 0004924
2009-04-27 10:41	garrigue	Assigned To	=> garrigue
2009-04-27 10:41	garrigue	Status	new => confirmed
2009-05-19 07:44	garrigue	Note Added: 0004956
2009-05-19 07:44	garrigue	Status	confirmed => resolved
2009-05-19 07:44	garrigue	Fixed in Version	=> 3.11.1+dev
2009-05-19 07:44	garrigue	Resolution	open => fixed
2009-05-19 10:25	garrigue	Note Added: 0004958
2011-05-29 12:14	xleroy	Status	resolved => closed

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