[ruby-core:97297] [Ruby master Bug#16660] Struct#deconstruct_keys inconsistent behavior

Issue #16660 has been reported by palkan (Vladimir Dementyev).

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Bug #16660: Struct#deconstruct_keys inconsistent behavior 
https://bugs.ruby-lang.org/issues/16660

* Author: palkan (Vladimir Dementyev)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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Here is an example of a kind of surprising (at least to me) `Struct#deconstruct_keys` behaviour:

```ruby
klass = Struct.new(:a, :b)
s = klass.new(1, 2)
```

1) When some keys are not recognized and the total number of the keys is not greater than the size of the struct:

```ruby
s.deconstruct_keys([:a, :c])
#=> {a: 1}
```

2) When some keys are not recognized but the total number of the keys is greater than the size of the struct:

```ruby
s.deconstruct_keys([:a, :b, :c])
#=> {}
```

It's not clear why the first one filters unknown keys and the latter one returns an empty Hash instead of the `{a: 1}`.

This behaviour was introduced in https://github.com/ruby/ruby/commit/2439948bcc0ec9daf91cf79301195e59bad49aff. Prior to that change an empty hash was returned in both cases.






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