Issue #6048 has been reported by Marc-Andre Lafortune.

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Bug #6048: {Unbound}Method#hash doesn't always return the right value
https://bugs.ruby-lang.org/issues/6048

Author: Marc-Andre Lafortune
Status: Open
Priority: Normal
Assignee: Marc-Andre Lafortune
Category: core
Target version: 2.0.0
ruby -v: r34632


{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
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Marc-André


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