[ruby-core:98086] [Ruby master Feature#13820] Add a nil coalescing operator

Issue #13820 has been updated by bsarrazin (Ben Sarrazin).


Kotlin has this feature, Swift has this feature, many other languages have this feature.
Ruby _needs_ this feature :D

```ruby
"a truthy value" || foo("something else") # The operator also short circuits so the method `foo` will never even get called
false || foo("something else") # Left side is falsy, so evaluate the right side, but is often not the intent
nil || foo("something else") # Nil is also falsy, so evaluate the right side

"a truthy value" ?? foo("something else") # String is true and not nil, so nothing changes here
false ?? foo("something else") # This changes. Left side is not nil, so the right side is never evaluated
nil ?? foo("something else") # Like with `||`, left side is nil so evaluate the right side

# so this "does the right thing", as far as my maybe not great example goes
https = opts[:https] ?? true
```

This is exactly why Ruby needs this feature. The lack of type safety, combined with the fact that many tools are taking "false" string as input is compelling argument to add this to the language.

----------------------------------------
Feature #13820: Add a nil coalescing operator
https://bugs.ruby-lang.org/issues/13820#change-85322

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