Issue #20509 has been updated by matz (Yukihiro Matsumoto). It is intentional behavior. Usually, having `to_ary` means the object must behave as an array. If the object `a` is an array and the object `b` responds to `to_ary`, I expect the same result from `b == a` and `a == b.to_ary`. We use the former to reduce unnecessary object allocation. Same for `to_hash` respectively. Maybe we should document this expectation clearly in the reference. Matz. ---------------------------------------- Misc #20509: Document importance of #to_ary and #to_hash for Array#== and Hash#== https://bugs.ruby-lang.org/issues/20509#change-109320 * Author: gettalong (Thomas Leitner) * Status: Open ---------------------------------------- Both `Array#==` and `Hash#==` provide special behaviour in case the `other` argument is not an Array/Hash but defines the special `#to_ary`/`#to_hash` methods. Those methods are never called, they are just checked for existence. And if they exist, `other#==` is called to allow the `other` argument to decide whether the two objects are equal. I think this is worth mentioning in the documentation for `Array#==` and `Hash#==`. [Background: In my PDF library HexaPDF I have defined two classes `PDFArray` and `Dictionary` which act like Array and Hash but provide special PDF specific behaviour. For PDFArray I defined the `#to_ary` method but for Dictionary just the `#to_h` method. I have come across a bug where comparing Arrays with PDFArrays just works as it should be but comparing Hashes with Dictionaries doesn't due to the absence of `#to_hash` (it seems I removed `Dictionary#to_hash` in 2017 due to problems with automatic destructuring when passing a Dictionary as argument; from what I see that should be no problem anymore, so I will just add it back).] -- https://bugs.ruby-lang.org/