Issue #19712 has been updated by matz (Yukihiro Matsumoto). The `#reopen` has special semantics that re-initialized the IO object. As a result, singleton methods will be wiped out. So I propose to update the document to describe regarding singleton classes (and it may change the class of the receiver). Matz. ---------------------------------------- Feature #19712: IO#reopen removes singleton class https://bugs.ruby-lang.org/issues/19712#change-103458 * Author: itarato (Peter Arato) * Status: Open * Priority: Normal ---------------------------------------- The documentation states:
This may dynamically change the actual class of this stream.
As well `#reopen` removes the singleton class, even when the logical class is the same. This can be surprising at times. An example: ``` ruby io = File.open(__FILE__) io.define_singleton_method(:foo) {} io.reopen(File.open(__FILE__)) io.foo # `<main>': undefined method `foo' for #<File:/test.rb> (NoMethodError) ``` An example where this was an issue: https://github.com/oracle/truffleruby/pull/3088 Tl;dr: a popular gem was enhancing the singleton class of STDOUT, while Rails/ActiveSupport was executing an `IO#reopen` - invalidating the changes by the gem. While it's hard to trivially solve this with keeping the intended functionality, could it be considered to make edge cases more visible? Examples: - `IO#reopen` issues a warning message when the receiver's singleton class has been updated (but keep removing it as of current state) - `IO#reopen` is limited on instances with a default singleton class, and issues an error when called on an instance with an updated singleton class - make `IO#reopen` carry over the singleton class *only* if the recipient and argument's class are the same, and yield an error otherwise (different classes) These are just ideas. I'm more looking for feedback from the core devs at this point. Thanks in advance! -- https://bugs.ruby-lang.org/