Analyzer should trigger an error if enum comparison is used as const expression

[a14n]

With Dart 2.13.0 (build 2.13.0-150.0.dev) the analyzer accepts enum comparison in const expression though enum comparisons are not yet allowed in asserts.

enum E { a, b }
@immutable
class A {
  const A([E e = E.b]) : assert(e != E.a); // no diagnostic but there should be an `invalid_constant` error for `e != E.a`
}
void main() {
  const A(); // const_evaluation_throws_exception
}

/cc @scheglov @bwilkerson

[srawlins]

I've seen a few bugs about analyzer accepting code in assert initializers, I think also with a != operation...

[eernstg]

It's on my TODO list to push on dart-lang/language#1811, which would turn e == E.a and e != E.a into constant expressions (and indeed e1 == e2 as well as e1 != e2 for any e1 that has primitive equality). So please don't rush to change the implemented behavior: It seems to be a step away from a behavior which is likely to be the specified behavior in the near future.