Spec does not forbid using a non-const superconstructor from a const constructor

[rakudrama]

I can't find anything prohibiting the problem in issue #11624.

Section "7.6.3 Constant Constructors" does not appear to restrict the superconstructor used by a const constructor's initializers.
I did not find a restriction elsewhere.

[gbracha]

Added Accepted label.

[DartBot]

This comment was originally written by @mhausner

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I spoke with Gilad about this. The simple rule 'a const constructor can only call const super constructors" does not work because class Object does not have a const constructor. This is a problem because a missing super call in the initializer list implies a call to the nullary constructor of the super class.

class Point {
  final x, y;
  const Point(this.x, this.y); <<< implicit call to non-const constructor Object().
}

We have several options:

1. We can add an exception that a super call to the nullary non-const constructor of class Object is allowed.
2. We can define the (nullary) constructor of class Object to be const.

In the case of 2), the consequence is that 'const Object()" is legal and must return a canonicalized instance of type Object.

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cc @kasperl.
cc @larsbak.
cc @iposva-google.
cc @mhausner.

[DartBot]

This comment was originally written by @mhausner

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As it turns out, our core library already defines the constructor of class Object to be const. The spec does not describe the interface of Object, so it may not have to mention this fact.

/**
 * Everything in Dart is an [Object].
 /
class Object {
  /**
    Creates a new [Object] instance.
   *
   * [Object] instances have no meaningful state, and are only useful
   * through their identity. An [Object] instance is equal to itself
   * only.
   */
  const Object();
  ...

[gbracha]

I have added the requirement that the superinitializer used in a constant constructor (implicitly or explicitly) must denote a const constructor of the superclass.

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Added Done label.