From: "shugo (Shugo Maeda) via ruby-core" <ruby-core@...>
Date: 2026-10-22T09:55:23+00:00
Subject: [ruby-core:127210] [Ruby Bug#22462] Method#== can return false for methods that should be considered equal

Issue #22462 has been updated by shugo (Shugo Maeda).


As I commented in https://github.com/ruby/ruby/pull/19474#pullrequestreview-5609342827, with the PR change, `==` UnboundMethods can bind to non-`==` Methods:

```ruby
module M; def m = [:M, *super]; end
class Base; def m = [:Base]; end
class A < Base; include M; end
class B < A; prepend M; def m = [:B, *super]; end
b = B.new
ua = A.instance_method(:m)
ub = B.instance_method(:m)
p ua == ub                 # => true
p ua.bind(b) == ub.bind(b) # => true before, false after
```

I suggested there normalizing an iclass to its module in `method_eq()`, but Method objects with different iclasses may behave differently when `super` is called.

Another option would be to reconsider [Feature #18798].

If `UnboundMethod#==` compared iclasses, `==` would imply the same behavior, but `ua == M.instance_method(:m)` would also be false, as would `Object.instance_method(:to_s) == Kernel.instance_method(:to_s)`. Treating an UnboundMethod obtained directly from the module as equal to one with any iclass of it would avoid this, but `==` would then not be transitive: `ua == M.instance_method(:m)` and `M.instance_method(:m) == ub` would both be true, while `ua == ub` would be false.

----------------------------------------
Bug #22462: Method#== can return false for methods that should be considered equal
https://bugs.ruby-lang.org/issues/22462#change-119625

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
When a `Method` is created using `UnboundMethod#bind` and `#super_method`, the order in which the `#bind` and `#super_method` were applied could result in `Method` objects that were considered unequal.

I believe we should have the invariant: for any `UnboundMethod` `um` where `super_method` is not `nil` and `o` is a valid object to bind to `um`, the following code should be true:

```ruby
um.bind(o).super_method == um.super_method.bind(o)
```

However, that didn't work correctly in some cases:

```ruby
module M; def m = [:M, *super]; end
module N; include M; end
class C; def m = :C; end
class SC < C
  include N
  prepend M
  def m = [:SC, *super]
end
o = SC.new
um = SC.instance_method(:m).super_method.super_method  # M#m below SC (included via N)
um.bind(o).super_method == um.super_method.bind(o)
# => false # should be true
```

There is a similar invariant for Method: for any `Method` `m` where `super_method` is not `nil`, the following code should be true:

```ruby
m.unbind.super_method == m.super_method.unbind
```

This appears to already hold, as the existing refinement tests already checked for this behavior.

I've submitted a PR to fix this issue: https://github.com/ruby/ruby/pull/19474



-- 
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