[ruby-core:98002] [Ruby master Bug#16797] Array#flatten not checking `respond_to? :to_ary`

Issue #16797 has been updated by UlyssesZhan (有丘 詹).


nobu (Nobuyoshi Nakada) wrote in #note-1:
> Read the error message.
> 
> ```
> -:4:in `flatten': can't convert Object to Array (Object#to_ary gives Object) (TypeError)
> ```

Read what I wrote:
> It should check whether an object responds to to_ary before trying to convert it into an array.

I know it used `to_ary` method and got an unexpected result, but in fact calling `a.to_ary` itself is unexpected because `a.respond_to? :to_ary` is `false`.

Detecting `NoMethodError` is a bit silly.

----------------------------------------
Bug #16797: Array#flatten not checking `respond_to? :to_ary`
https://bugs.ruby-lang.org/issues/16797#change-85231

* Author: UlyssesZhan (有丘 詹)
* Status: Rejected
* Priority: Normal
* ruby -v: ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
```ruby
def (a = Object.new).method_missing(...)
  Object.new
end
[a].flatten # TypeError
```
It should check whether an object responds to `to_ary` before trying to convert it into an array.



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