[ruby-core:102895] [Ruby master Bug#17725] Prepend breaks ability to override optimized methods

Issue #17725 has been updated by nobu (Nobuyoshi Nakada).

Subject changed from Prepend Breaks Ability to Alias to Prepend breaks ability to override optimized methods

Alias is not concerned.

```ruby
# bug-17725.rb
String.prepend(Module.new) unless ARGV.empty?
class String
  def + other
    'blah blah'
  end
end

p 'a' + 'b'
```

```sh
$ ruby bug-17725.rb 
"blah blah"

$ ruby bug-17725.rb 1
"ab"
```

Seems the redefinition flag is not set by prepend.

----------------------------------------
Bug #17725: Prepend breaks ability to override optimized methods
https://bugs.ruby-lang.org/issues/17725#change-90955

* Author: joshuadreed (Josh Reed)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
Prepending any module to `String` and `Hash` (and possibly other or all classes?) blocks the ability to `alias` 

As an example:
``` ruby
module Dummy; end
# String.prepend(Dummy)
class String
  alias_method(:old_plus, :+)
  def + other
    puts 'blah blah'
    old_plus(other)
  end
end

'a' + 'b'

> blah blah
```

``` ruby
module Dummy; end
String.prepend(Dummy)
class String
  alias_method(:old_plus, :+)
  def + other
    puts 'blah blah'
    old_plus(other)
  end
end

'a' + 'b'

>
```

Prepending after an `alias` does not affect the previous `alias`



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