[ruby-core:82600] [Ruby trunk Bug#13855] Hash#compact! returns nil if the hash is empty

Issue #13855 has been updated by duerst (Martin Dürst).

Assignee set to nobu (Nobuyoshi Nakada)

`nil` is returned if there is no change, not only if the hash is empty:

```
hash = { foo: :bar, one: :two } # => {:foo=>:bar, :one=>:two}
hash.compact!                   # => nil
```

This is consistent with many other bang methods. The idea is that the result is available in the changed variable already, and the return value can be used in an `if` or `while` expression.

The patch looks good, I hope Nobu can apply it (I can't automatically integrate patches from github).

----------------------------------------
Bug #13855: Hash#compact! returns nil if the hash is empty
https://bugs.ruby-lang.org/issues/13855#change-66423

* Author: elandesign (Paul Smith)
* Status: Open
* Priority: Normal
* Assignee: nobu (Nobuyoshi Nakada)
* Target version: 
* ruby -v: ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-darwin16]
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
This behaviour feels like a bug to me.

From the documentation (with my emphasis):

> compact! → hsh
> Removes all nil values from the hash. **Returns the hash**.

However if the hash contains no keys, the method returns nil.

~~~
irb(main):001:0> {}.compact!
=> nil

# For Comparison
irb(main):002:0> { foo: nil }.compact!
=> {}
irb(main):003:0> {}.compact
=> {}
irb(main):004:0> { foo: nil }.compact
=> {}
~~~



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