Dart does not throw compile time error when wrong type is assigned to a variable using the if null operator (??)

[gtrochimiuk]

Dart VM version: 2.3.0-dev.0.5.flutter-a1668566e5 (Tue Apr 30 20:35:41 2019 +0200) on "macos_x64"

Dart does not throw compile time error when wrong type is assigned to a variable if you use the "if null" operator. Following example compiles without error and then throws a runtime error:

class ExampleClass {}

void main() {
  ExampleClass example = 'string' ?? ExampleClass();
}

[lrhn]

I believe this is a classical example of the way our unification interacts with implicit downcasts.

The static type of 'string' ?? ExampleClass() is the least supertype of String and ExampleType, which is Object. Since Object is a supertype of ExampleClass, the assignment is allowed statically as an implicit downcast, and it fails at run-time.

With non-nullable types, the 'string' ?? ... will likely become either a warning or an error (the expression cannot be null, so the ?? is irrelevant), and when implicit downcasts are disallowed, that particular error will be caught as well.

Until then, the analyzer option implicit-casts can warn you about all downcasts, which will catch cases like this ... and a lot of other downcasts which are probably perfectly fine.

[gtrochimiuk]

I see, thank you for the explanation.

[natebosch]

Closing since NNBD will also disable implicit casts and there are no specific actions to take on this issue.

dart-lang/language#192