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Problem with forall and method #5322

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vicuna opened this issue Jul 28, 2011 · 1 comment

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commented Jul 28, 2011

Original bug ID: 5322
Reporter: poluxmoon
Status: closed (set by @garrigue on 2011-07-29T01:56:43Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 3.12.0
Fixed in version: 3.13.0+dev
Category: ~DO NOT USE (was: OCaml general)

Bug description

Type error on code similar to :


type 'par t = 'par

let ident v = v

class alias =
object
method alias : 'a . 'a t -> 'a = ident (* type error here *)
end

Which gives :


Error: This expression has type 'a. 'a t -> 'a
but an expression was expected of type 'b. 'b t -> 'b
Type 'a a = 'a is not compatible with type 'b t = 'b

Additional information

The problem is related to unification of foralls containing named type in a method.
The compiler accepts equivalent code when not using the named type "t" or when not using a method (see attached file).

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commented Jul 29, 2011

Comment author: @garrigue

Thanks for your report.
Both Ctype.unify3 and Ctype.moregen didn't consider the possibility that a type abbreviation may expand to a universal type variable.
This is now fixed in both trunk and 3.12 branch (revision 11159).

@vicuna vicuna closed this Jul 29, 2011

@vicuna vicuna added the bug label Mar 20, 2019

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