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Bug #6048

closed

{Unbound}Method#hash doesn't always return the right value

Added by marcandre (Marc-Andre Lafortune) about 12 years ago. Updated about 12 years ago.

Status:
Closed
Target version:
ruby -v:
r34632
Backport:
[ruby-core:42755]

Description

{Unbound}Method#hash doesn't always return the right value.

map, collect = Array.instance_method(:map), Array.instance_method(:collect)

map.eql?(collect)         # => true
map.hash == collect.hash  # => false

I'm tempted to think that this is an obvious bug with an obvious solution but let me state:

As per the documentation and the design of hash tables, if two objects are eql? then they must have the same hash.

Either map should not be eql? to collect or else their hash should be the same.

As they are identical methods, it is correct that they are eql?, so hash must return the same value.

My proposed behavior passes my strict superiority test and is also "straightforward" as I could find no intent for the current behavior.

One solution is to ensure that all aliased methods are defined using rb_define_alias, which appears to yield the same hash. Another is to compute the hash correctly. Maybe there is a third approach.

I'm not super confident I took the right approach, but here is a patch for the second one. I think it is more robust and more consistent with how {Unbound}Method.eql? is implemented. It may also fix other cases of mismatch between eql? and hash, I didn't investigate further.

I'd be grateful if someone could review the patch (Koichi?) and let me know if I took the right approach and if I put things in the right place.

Thanks

Marc-André


Files

method_hash.patch (4.32 KB) method_hash.patch marcandre (Marc-Andre Lafortune), 02/20/2012 03:19 PM
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