[ruby-core:82581] [Ruby trunk Feature#13820] Add a nill coalescing operator

Issue #13820 has been updated by t3hk0d3 (Igor Yamolov).


I'd would also add following use-case - (lazy) boolean caching:

```ruby
def stuff_working?
   @stuff_working ||= check_stuff_working # 
end

private
def check_stuff_working
   # expensive computation or external (HTTP) request
   ... 
   return false # or true
end
```

----------------------------------------
Feature #13820: Add a nill coalescing operator
https://bugs.ruby-lang.org/issues/13820#change-66403

* Author: williamn (William Newbery)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
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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