augmented() in generative constructors needs additional compile-time checks

[eernstg]

The analyzer currently (ac9b6d1) does not report a compile-time error for the following program, in spite of the fact that augmented() is specified to have type void:

class A {
  A x;
  A() {
    print('Running introductory!');
  }
  augment A(): x = augmented();
}

Also, the expression augmented() should evaluate to null, which means that it would be a soundness violation to allow it to be used as the initial value of x.

Another program which is accepted by the analyzer with no compile-time errors is the following:

class A {
  dynamic x;
  A() {
    print('Running introductory!');
  }
  augment A(): x = augmented();
}

In this case I believe there must also be an error (this is discussed in dart-lang/language#4039), even though the above mentioned soundness violation does not exist.

A similar situation exists with a redirecting generative constructor where this(augmented()) does not cause any compile-time errors.

class B {
  B(Object? o);
  B.named(): this(42);
  augment B.named(): this(augmented());
}

In this case there should be an error because an expression of type void (namely augmented()) occurs in a location where an Object? is expected, and it is not on the allowlist for using the value of an expression of type void.