[ruby-core:102662] [Ruby master Bug#16649] Object with [] method doesn't work as String#encode's fallback

Issue #16649 has been updated by usa (Usaku NAKAMURA).

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

backported into ruby_2_6 at r67901.

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Bug #16649: Object with [] method doesn't work as String#encode's fallback
https://bugs.ruby-lang.org/issues/16649#change-90656

* Author: pocke (Masataka Kuwabara)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-02-23T15:28:34Z master f88d209bb7) [x86_64-linux]
* Backport: 2.5: REQUIRED, 2.6: DONE, 2.7: DONE
----------------------------------------
`String#encode` document says fallback can be a Hash, a Proc, a Method or an object which has `[]` method.

>  Sets the replacement string by the given object for undefined character. The object should be a Hash, a Proc, a Method, or an object which has [] method
> https://docs.ruby-lang.org/en/2.7.0/String.html#method-i-encode

But actually an object with `[]` does not work.


Reproduce:


```ruby
class C
  def [](str)
    '!'
  end
end

str = "\u00b7\u2014"
p str.encoding # => #<Encoding:UTF-8>

# It works
p str.encode("Windows-31J", fallback: C.new.method(:[]))
# => "!!"

# It does not work
p str.encode("Windows-31J", fallback: C.new)
# => U+00B7 from UTF-8 to Windows-31J (Encoding::UndefinedConversionError)
```




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