Issue #15596 has been updated by nobu (Nobuyoshi Nakada).


It is a natural behavior.

This method prints multiple arguments to `STDERR`, per line.
`warn nil` passes one argument and prints one line, whereas `warn` passes no argument  and prints zero line.

----------------------------------------
Bug #15596: Kernel.warn without arguments should do the same as Kernel.warn(nil)
https://bugs.ruby-lang.org/issues/15596#change-76755

* Author: kke (Kimmo Lehto)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.1p33 (2019-01-30 revision 66950) [x86_64-darwin18]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
Kernel.warn without arguments does not print an empty line to $stderr.

This is inconsistent with `Kernel.puts` and it feels weird, because it does not act like a regular Ruby method would (if it was written in Ruby instead of C, it would probably be defined like `def warn(msg = nil)` and calling it with or without nil as argument would make no difference)

### Expected behavior:

```
irb(main):001:0> warn nil

=> nil
irb(main):002:0> warn

=> nil
```

### Actual behavior:

```
irb(main):001:0> warn nil

=> nil
irb(main):002:0> warn
=> nil
```






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