[ruby-core:82407] [Ruby trunk Feature#13820] Add a nill coalescing operator

Issue #13820 has been updated by phluid61 (Matthew Kerwin).


In perl I find `$x // $y` useful vs `$x || $y` because sometimes you want to accept `""` and `0` as values.

In ruby the only 'defined' falsey value is `false`, so I'm not sure how useful this feature is here.

If you're pulling options from a hash, for example, it's probably better to use a signal like `h.fetch k, d` to show that you accept falsey values from the hash, and/or `x.nil? ? y : x` to show that you explicitly only don't want `nil`

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Feature #13820: Add a nill coalescing operator
https://bugs.ruby-lang.org/issues/13820#change-66207

* Author: williamn (William Newbery)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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It would be nice if Ruby had an operator that only considered `nil` as false, like the null coalescing operators or "Logical Defined-Or operator" (Perl) found in some other languages. Ive seen things like `//` and `//=`m `??` and `??=`, or `?:` used for this.

This would work like `||` and `||=` for short circuiting etc. except that only `nil` is considered a false condition.

While Ruby considers only "false" and "nil" as false, with everything else true ("", [], {}, etc.) I still find occasionally people trip up when using logical or, `||` and `||=` when the value may be false.


```ruby
a = 0     || 55 # = 0 Ruby already considers 0, "", etc. as true (oter languages do differ a lot here)
a = 0     ?? 55 # = 0 So no change here
a = nil   || 55 # = 55, nil is false so right side is evaulated.
a = nil   ?? 55 # = 55, again no change
a = false || 55 # = 55, however false is false for logical or
a = false ?? 55 # = false, but its still a non-nil value
```


For example when doing things like:

```ruby
def lazy
  @lazy ||= compute_this
end


def fetch(id, **opts)
  host  = opts[:host] || default_host
  https = opts[:https] || true
  port  = opts[:port] || (https ? 443 : 80)
  ...
```

Normally the intention is to use a default value or compute an action if no value is provided, which if the value may be false then requires special handling, or sometimes is missed and results in a bug.



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