dart2js considers ints to be doubles

[trinarytree]

in the dart vm, 1 is double is false.
but in dart2js, 1 is double is true.
according to dart's type hierarchy, int is not a subtype of double, so this seems like a dart2js bug.

here's 1 way this can manifest as trouble downstream:
someone sees a function f that returns the output of double.parse.
double.parse declares that it returns a double, but this friendly soul observes that sometimes
double.parse actually outputs an int (e.g. here not realizing that dartpad uses dart2js semantics) and so changes the return type of f to num because they were afraid that some client
might receive an int, think it's a double, and induce a bug.
this forces all consumers of f to update their code, and so on.

now int and double both extend num, and double mysteriously has no extra methods.
or, to look at it from another perspective, it's mysterious that num has methods that
seem more appropriate for doubles than all numbers, like round, truncate.
so an int can duck type as a double.
perhaps this is how dart2js can mostly get away with this,
but woe to anyone writing this code:

num numScoops({bool userLikesIcecream}) => userLikesIcecream ? 1.5 : 1;

void main() {
  var n = numScoops(userLikesIcecream: false);
  print(n is double
      ? "here's a little extra for you!"  // in dart2js, user gets confused
      : "icecream's all we got");
}

[sigmundch]

@trinarytree - unfortunately this is a bug that cannot be fixed in dart2js.

Take a look at #1533 for more details.

[sigmundch]

FWIW, I agree with you that this surprising behavior is not ideal, so I hope the language can be improved to highlight warn about this. For example, maybe there should be a warning if code has checks for is double and is int, since we know that they might have inconsistent semantics based on the target platform.

@floitschG @munificent - are there plans to address this in the language?

[munificent]

No concrete plans that I am aware of though I personally would like to do a better job of helping users deal with cases like this.

[zoechi]

What about a warning in dart2js about using is double or is int?

[munificent]

I thought it had that warning, but maybe it disappeared at some point?