`Array#flatten` ignores `respond_to_missing?` when calling `to_ary`

description.md

This issue was reported on bugs.ruby-lang.org: https://bugs.ruby-lang.org/issues/11465

The issue appears when using Array#flatten on an instance of Array containing instances of a Class that implements method_missing. Ruby still calls #to_ary even when said class implements respond_to_missing? and returns false for to_ary. Everything works when the class also overwrites respond_to?.

Questions:

1. Is it expected that respond_to_missing? has no effect in the scenario above?
2. Should a possible workaround rather define a respond_to? method or overwrite to_ary and return nil?

Context:

The problem was found when working on a issue reported on the Rails tracker: rails/rails#21296

reproduction.rb

class RespondToMissing
  def respond_to_missing?(method, stuff)
    return false if method == :to_ary
    super
  end

  def method_missing(*args)
    super
  end
end

class RespondTo
  def respond_to?(method)
    return false if method == :to_ary
    super
  end

  def method_missing(*args)
    super
  end
end

trace = TracePoint.new(:raise) do |tp|
  p tp.raised_exception
  puts tp.raised_exception.backtrace.join("\n")
  puts
end
trace.enable

p RespondToMissing.new.respond_to? :to_ary # => false
p RespondTo.new.respond_to? :to_ary # => false

[RespondTo.new].flatten
# no internal exceptions are raised

[RespondToMissing.new].flatten
# #to_ary is being called and internal NoMethodError is raised.

# OUTPUT:
# false
# false
# #<NoMethodError: undefined method `to_ary' for #<RespondToMissing:0x007fd57322ce78>>
# script.rb:10:in `method_missing'
# script.rb:38:in `flatten'
# script.rb:38:in `<main>'