Sdk: Switch statement and `runtimeType`

[mdebbar]

I was looking at https://github.com/dart-lang/language/issues/83 and https://github.com/dart-lang/language/issues/79 to see if type promotion would be useful inside switch statements when I stumbled upon this bug:

class Cat {
  String meow() => 'meow';
}

String foo(dynamic a) {
  switch(a.runtimeType) {
    case String: return 'str';
    case int: return 'int';
    case Cat: return a.meow();
    default: return 'unknown';
  }
}

void main() {
  print(Cat().runtimeType == Cat); // true ✅
  print(foo('abc')); // 'str' ✅
  print(foo(Cat())); // 'unknown' (why not 'meow' ??)
}

I only tested this on DartPad.

[lrhn]

It's a bug in dart2js. Or two bugs, actually.

First if all, you should not be allowed to use Cat as a switch case expression. Switch case expressions must be constants that do not override Object.== (or integers, strings or certain constant Symbol instances). The Type object overrides Object.==. If you check print(identical(Cat().runtimeType, Cat)); you get false, so it has to override equality to return true for the == check.

Second, the dart2js implementation of switch assumes that you can use identity for checking (since the values should not override Object.==), but since Symbol also overrides ==, that is a false assumption. Even if we decided to allow Type instances as switch case expressions, dart2js would still need to fix this.

(I'm filing this as a dart2js error for the switch case using identity since allowing Type objects is a front-end missing error case, and possibly already known).

So, example:

main() {
  switch(Symbol("foo")) {
    case #foo: print("yes"); break;
    default: print("no");
  }
}

This prints "no" in dart2js. It must print "yes". If we ever allow Type objects as switch case expressions, that too must use == for comparison.

[lrhn]

Since all implementations allow it anyway, we now intend to formally allow Type objects as switch case expression. That still means that dart2js is doing it wrong.

[stegrams]

Hello from 2020 :+1:
The state has not change yet but a, maybe sloppy, solution for both above examples would be:

class Cat {
  String meow() => 'meow';
}

String foo(dynamic a) {
  switch (a.runtimeType.toString()) {
    case 'String':
      return 'str';
    case 'int':
      return 'int';
    case 'Cat':
      return a.meow();
    default:
      return 'unknown';
  }
}

void main() {
  print(foo('abc')); // 'str' ✅
  print(foo(42)); // 'int' ✅
  print(foo(Cat())); // 'meow' ✅

  // prints 'yes' ✅
  switch (Symbol("foo").toString()) {
    case 'Symbol("foo")':
      print("yes");
      break;
    default:
      print("no");
  }
}

[rakudrama]

dart2js has a new implementation of runtime Type objects that are canonicalized.
The Type implementation no longer overrides ==.
This means that the testing using identical() on the switch cases is now a correct implementation for Type objects (The issue with Symbols might remain).

However, this does not mean that the original program will behave as expected. The types int, List, String etc are _interfaces_. There is no guarantee on the type returned by, say, 5.runtimeType.
dart2js returns a Type instance for the _implementation_ type, which is called JSInt.

[lrhn]

With ahead-of-time compilation, we cannot really assume anything about compiler-provided implementation types. Even if dart2js canonicalizes types, the compiler front-end won't know that, and the same code might fail when compiled for a different back-end.

That's why we special-cased constant Type objects created from type literals in the specification, so it doesn't actually matter whether they override ==.

(That is really a hole in the specification - it talks about creating objects at compile-time, but that only makes sense for classes which are available at compile-time, not those which are provided by the run-time system. In fact, the specification kind-of assumes that it does know the whole program, which just isn't true).

We make no promises that integer values return int as their runtimeType. The VM integer implementations do (by overriding runtimeType to return int), but dart2js has the problem that 5.0 is both an int and a double, so it cannot pretend to be only int or double.

I recommend never using runtimeType for anything. Just use subtype checks. If you do use runtimeType, only use it on classes that you have written yourself, so you know that they are providing the correct value.

[sigmundch]

Now that the new RTI is out we believe the Cat case in the original post will start working. I don't believe there are any additional action items on our end, so I'll close this issue.

I want to emphasize @lrhn's advice. It's best to not rely on runtimeType for almost anything. In addition, its important to be cautious about .runtimeType.toString() - the resulting String is allowed to change and dart2js will use a different string under minification. (e.g. you may get "minified:xQ" instead of "Cat").