[ruby-core:120011] [Ruby master Feature#13820] Add a nil coalescing operator

Issue #13820 has been updated by genezys (Vincent Robert).


Here are my 2 cents regarding this subject, if it can help moving forward with this feature.

Wkipedia has a great article explaining and listing null coalescing operators in other languages: https://en.wikipedia.org/wiki/Null_coalescing_operator
Here is a summary of all operators ordered by usage:

- `??`: ATS, C#, JavaScript, PHP, PowerShell, Swift
- No operator: Python, Ruby, Rust, SQL
- `?:`: CFML, Kotlin, Objective-C
- `!`: Freemarker, Haskell
- `:-`: Bourne-like shells 
- `|?`: F#
- `//`: Perl
- `%||%`: R
- `If`: VB.NET

All languages that do not provide an operator offer a way to perform null coalescing. 
In Ruby, you could add a `coalesce` method with a special implementation on NilClass:

```
class Object
  def coalesce(value = nil)
    self
  end
end

class NilClass
  def coalesce(value = nil)
    block_given? ? yield : value
  end
end

> "a".coalesce("b") # => "a"
> "a".coalesce { "b" } # => "a"
> nil.coalesce("b") # => "b"
> nil.coalesce { "b" } # => "b"
> false.coalesce(true) # => false
> nil.coalesce(true) # => true
```

Keep in mind that this does not address the issue of the null coalescing assignment operator (aka `??=`)

----------------------------------------
Feature #13820: Add a nil coalescing operator
https://bugs.ruby-lang.org/issues/13820#change-110753

* Author: williamn (William Newbery)
* Status: Open
----------------------------------------
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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