Different behavior in web and VM when casting int to double

[talski]

From dart-lang/language#20 and #34355

double? doubleValue;
dynamic integerValue = 1;

doubleValue = integerValue;

On windows I get type 'int' is not a subtype of type 'double?'.
On web I got no errors.

Dart SDK version: 2.14.0-383.0.dev on "windows_x64"

[lrhn]

This is working as intended.

On the web, when compiled to JavaScript, all numbers are JavaScript numbers — which means that they are all doubles.

Even integers are represented by JavaScript numbers, so 1 and 1.0 is the exact same object, which is then treated as if it was an instance of a class which implements both int and double.

On native, 1 and 1.0 are different objects and instances of different classes.

The initialization dynamic integerValue = 1; treats 1 as an integer literal because it's not in a double context. That makes it an integer value on native, and the implicit integerValue as double? fails.

On the web, dynamic integerValue = 1; also treats 1 as an integer literal, which happens to evaluate to the same object as 1.0.
When you do the the integerValue as double? which is implicit in the downcast in the assignment, the web runtime system checks whether the value, 1 aka 1.0, implements double?, and it is (also) a double so the case succeeds.