Issue #13855 has been reported by elandesign (Paul Smith).

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

* Author: elandesign (Paul Smith)
* Status: Open
* Priority: Normal
* Assignee: 
* 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
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This behaviour feels like a bug to me.

From the documentation (with my emphasis):

> compact! вк 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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