Cannot have a const constructor even if members are const-constructible?

[jamesderlin]

We'd like to have code like:

class Foo {
  const Foo(this.value);

  final int value;
}

class FooDelegate implements Foo {
  final Foo _foo;

  const FooDelegate(int value) : _foo = Foo(value); // Line 10

  @override
  int get value => _foo.value;
}

but this fails with:

error • Invalid constant value. • const_delegate.dart:10:41 • invalid_constant

We could remove const from the FooDelegate constructor, but without it FooDelegate is no longer perfectly substitutable for Foo (const foo = FooDelegate(1); would fail).

It would be nice if there were some way for a const constructor to const-construct members if possible.

[eernstg]

Cf. https://github.com/dart-lang/sdk/issues/20962.

[eernstg]

dart-lang/sdk#20962 was moved to this repo, where it should be; marking this as duplicate of its new location, #823, and closing.