if-case negation pattern variable outside of if (better early returns)

[Albert221]

I want to be able to use the if-case pattern matching, that when the if body returns, I'll have the access to pattern variable outside of the if. This will make early returning with if-cases possible. Consider the below:

bool method(Animal animal) {
  if (animal case final Cat cat) {
    cat.feed();
    cat.pet();
    cat.lifes = min(0, cat.lifes);
    return true;
  }

  return false;
}

Versus this, using if-case with a negated pattern match:

bool method(Animal animal) {
  if (animal case! final Cat cat) {
    return false;
  }

  cat.feed();
  cat.pet();
  cat.lifes = min(0, cat.lifes);
  return true;
}

The case! is an arbitrary syntax. I'd like any syntax for that.

Currently, to utilize early returns we need this:

bool method2(Animal animal) {
  final cat = animal;
  if (cat is! Cat) {
    return false;
  }

  cat.feed();
  cat.pet();
  cat.lifes = min(0, cat.lifes);

  return true;
}

Java has such feature, you can do if (!(animal instanceof final Cat cat)) - https://docs.oracle.com/en/java/javase/17/language/pattern-matching-instanceof.html#subhead1

[Albert221]

Ah it was already proposed, but closed as a duplicate: #3865

[lrhn]

This sounds like it can be generalized to "case expressions".

If expr case pattern [when expr] was an expression (likely one that needs parentheses in every non-trivial context), and its bindings were available on the true branch, just like promotions would be, then you could just write:

if (!(e case var v?)) {
  return;
}
v.something();

You could possibly also write

if (o.hasBanana && (o.banana case pattern)) {
  // ...
}

or

(e case var p?) ? p.something() : 0

which are currently not allowed.

That would be #3059.