Passing a non-const value to a const constructor gives a confusing error message

[Hixie]

Consider this code:

class Foo {
  const Foo(this.x);
  final int x;
}

void test(int x) {
  const Foo(x);
}


void main() { }

I would expect one error message, namely, "Arguments of a constant creation must be constant expressions" or some variant thereof. However, today we get two errors, and the first one is really confusing:

  error • A value of type 'Null' can't be assigned to a parameter of type 'int' in a const constructor • test.dart:7:13 • const_constructor_param_type_mismatch
  error • Arguments of a constant creation must be constant expressions • test.dart:7:13 • const_with_non_constant_argument

[khari998]

This is because const in Dart requires all values to be known at compile time. In this case, x is known at run-time, not compile-time because x is not known to the constructor until it is provided by the outer function.

So const Foo(x) doesn't work but const Foo(9) does. If you want to pass a parameter, just remove const.

If you're coming from a JavaScript background, const in Dart works much differently than it does in JavaScript.
final in Dart works more like const does in JavaScript. Here's a more thorough explanation:

https://stackoverflow.com/questions/50431055/what-is-the-difference-between-the-const-and-final-keywords-in-dart

[eggzotic]

This is because const in Dart requires all values to be known at compile time. In this case, x is known at run-time, not compile-time because x is not known to the constructor until it is provided by the outer function.

So const Foo(x) doesn't work but const Foo(9) does. If you want to pass a parameter, just remove const.

If you're coming from a JavaScript background, const in Dart works much differently than it does in JavaScript. final in Dart works more like const does in JavaScript. Here's a more thorough explanation:

https://stackoverflow.com/questions/50431055/what-is-the-difference-between-the-const-and-final-keywords-in-dart

I think you are missing the OP's point. While your answer is correct as to why that code is in error - the actual point is that the error message is super confusing, especially this part: "A value of type 'Null' can't be assigned to a parameter of type 'int' in a..." - why is Null mentioned? That is the part that could be improved. I am still getting this message today - and in my case it's within an enum (rather than a class) where the constructor must be const...