[ruby-core:103218] [Ruby master Bug#16931] `defined?` against a protected method call on an inherited instance returns wrong `nil`

Issue #16931 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

ruby_2_6 r67920 merged revision(s) de9d6a7a8cb0929e0b4cf0a3120971c7b8ee8927,da5bd0047d6c961105da210c69d4c88421324b70,ce91c5615ca96f357519043181fb5b76b57ee223,c53aebb1d2eb5afbb18f9b3db9f9c956a463a4e1,d05f04d27dd86c67e4a8dfff4392f806cf577bdf.

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Bug #16931: `defined?` against a protected method call on an inherited instance returns wrong `nil`
https://bugs.ruby-lang.org/issues/16931#change-91294

* Author: nobu (Nobuyoshi Nakada)
* Status: Closed
* Priority: Normal
* ruby -v: 2.8.0dev (2020-06-02T08:21:03Z master 56ca006784)
* Backport: 2.5: REQUIRED, 2.6: DONE, 2.7: DONE
----------------------------------------
The following code shows `:callable` first, but `nil` next, since 1.9.
If the method is callable, `defined?` expression has to return truthy value.

``` ruby
class A
  def foo
    :callable
  end
  protected :foo
  def t(x)
    x.foo
  end
  def q(x)
    defined?(x.foo)
  end
end
class B<A
end
a = A.new
b = B.new
p a.t(b) #=> :callable
p a.q(b) #=> nil
```




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