Default argument values should imply "const" context

[Hixie]

If you run this:

class Test {
  Test({ this.test = Duration(minutes: 1) }); // the default value is not explicitly "const"
  final Duration test;
}

void main() {
  Test();
}

...you get an exception (a bad one, currently, see dart-lang/sdk#35683).

I think the fact that only a constant is allowed to be used there should imply a constant context and thus make the "const" redundant and implied.

[eernstg]

It is by design not a constant context: The language team wanted to keep the door open for language extensions whereby default values could be ordinary expressions that would be computed (the tricky part: in which scope?) at invocation. This would turn into a breaking change if we had allowed C c = C() to mean C c = const C().

(You could say that we have also kept the door open for making it a constant context, should that turn out to be more useful.)

[srawlins]

This sounds like it is resolved?

[eernstg]

Well, considered as a language request we could still go two ways: (1) allow more general expressions (e.g., evaluated at each invocation) as default values, in which case it makes sense that this is not a constant context, or (2) decide that we won't do that (so default values must be constant), and then make it a constant context such that const can be omitted.

I think the proper thing to do would be to move it to the language repo. If a lot of people have a strong preference about this decision then this issue then they can use this issue to make it known.