Support type promotion with object patterns

[johnpryan]

I'm wondering if using an object pattern could promote the type in the same way an if statement does. For example, this code promotes the variable foo:

void main() {
  Object? foo = 123;
  if (foo is int) {
    print(foo / 10);
  } else {
    print(foo);
  }
}

But in Dart 3, a switch statement with an object pattern doesn't promote foo:

void main() {
  Object? foo = 123;
  switch(foo) {
    case int:
      print(foo / 10);
    default:
      print(foo);
  }
}

The operator '/' isn't defined for the type 'Object'.

[blaugold]

case int matches the constant Type value for int.

What you want to use is a variable pattern:

void main() {
  Object? foo = 123;
  switch(foo) {
    case final int foo:
      print(foo / 10);
    default:
      print(foo);
  }
}

[lrhn]

Another case for dart-lang/language#2911.

And as @blaugold points out, it does promote when the pattern actually checks the type.

[johnpryan]

Oops, I didn't realize I was matching the type int and not the value. This code works also:

void main() {
  Object? foo = 123;
  switch(foo) {
    case int():
      print(foo / 10);
    default:
      print(foo);
  }
}

[lrhn]

It's a very well known foot-gun in the new switch syntax. Everybody does that. Often more than once.

I personally blame type literals for the problem, not switches (dart-lang/language#2393 😉).

We plan to have a lint reminding you to do something else when you write case Type: (https://dart-lang.github.io/linter/lints/type_literal_in_constant_pattern.html).