[ruby-core:102351] [Ruby master Bug#17509] Custom respond_to? methods in modules break defined?(super)

Issue #17509 has been updated by naruse (Yui NARUSE).

Backport changed from 2.5: DONTNEED, 2.6: DONTNEED, 2.7: DONTNEED, 3.0: REQUIRED to 2.5: DONTNEED, 2.6: DONTNEED, 2.7: DONTNEED, 3.0: DONE

ruby_3_0 147453ad1e85bc94506c269e363e736675946908 merged revision(s) 85b5d4c8bf4cdcba4f1af65f2bc0c8ac716cb795.

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Bug #17509: Custom respond_to? methods in modules break defined?(super)
https://bugs.ruby-lang.org/issues/17509#change-90211

* Author: benediktdeicke (Benedikt Deicke)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin19]
* Backport: 2.5: DONTNEED, 2.6: DONTNEED, 2.7: DONTNEED, 3.0: DONE
----------------------------------------
When using `defined?(super)` to check that a superclass method exists before calling `super`, including modules with a custom `respond_to?` method seems to break the check so it wrongfully returns a truthy value, even though the superclass method doesn't exist.

This only happens on Ruby 3.0.0 and works fine on previous Ruby versions. This is possibly related to this change: https://github.com/ruby/ruby/pull/3777

### Example 

```ruby
module SomeModule
  def a_method
    defined?(super) ? super : :no_super_method
  end
end

module RespondModule
  def respond_to?(*)
    super
  end
end

class PlainClass
  include SomeModule
end

class ModuleClass
  include RespondModule
  include SomeModule
end

puts "PlainClass: #{PlainClass.new.a_method}"
puts "ModuleClass: #{ModuleClass.new.a_method}"
```

### Actual Result on Ruby 3.0.0p0

```
PlainClass: no_super_method
test.rb:3:in `a_method': super: no superclass method `a_method' for #<ModuleClass:0x00007fd77383f908> (NoMethodError)
Did you mean?  method
	from test.rb:24:in `<main>'
```

### Expected Result (like on older versions)

```
PlainClass: no_super_method
ModuleClass: no_super_method
```

Thanks to Rafael Fran軋 for helping me unravel this. 

---Files--------------------------------
test.rb (359 Bytes)


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