Wrong error in the analyzer in case of ternary operator and constant evaluation

[sgrekhov]

The code below produce an error in the analyzer and no issues in CFE

class C {
  static C one = const C(1); // Note, one is not a constant
  static const C two = const C(2);
  final int v;
  const C(this.v);
}

main() {
  const c = 2 > 1 ? C.two : C.one; // Analyzer error. Const variables must be initialized with a constant value.
}

There should be no this error in the analyzer.

cc @eernstg to confirm.

[eernstg]

It is true that this is how it is specified. However, I created the issue dart-lang/language#4311 in order to clarify the situation because this might well be unintended (all other rules maintain the property that every constant expression is also a potentially constant expression, but in this particular case it doesn't hold).

So we should probably put this issue on hold until that spec issue has been resolved.

[lrhn]

ACK.

We don't have a term for "syntactically a constant expression".

Being a potentially constant expression is a syntactic property, so we can say whether an expression is so without evaluating it.
Being a constant expression depends on how it evaluates.
Which means we have no way to express "would not be a constant expression if it was evaluated" — when it isn't evaluated.

If the text for ?/: in the constant section had said "It is further a constant expression if ..." then it would have to be a potentially constant expression too.
(It's not what we really want, or it's not enough, because for a constructor C(int x) : y = test ? const [x] : const [];, the const [x] is syntactically known to not be a valid constant expression, but the only way to see that is to check if it's a valid constant, which is undefined when it's not evaluated. (If it is evaluated, then it will fail because the elements of const[x] must beconstant expressions, and x is only potentially constant.)

So, not sure we have a way, in the current framework, to actually specify that some unevaluated expression is not constant.