CFE allows `!` selector in constants but shouldn't

[sgrekhov]

The below works well in VM but produces an error in the analyzer.

const answer = "42"!; // Analyzer error. Const variables must be initialized with a constant value.

main() {
  print(answer);
}

Constant expression allows ! selector, therefore, I believe, it's an analyzer issue.

@lrhn @eernstg please confirm.

Dart SDK version: 3.7.0-274.0.dev (dev) (Thu Dec 19 20:06:50 2024 -0800) on "windows_x64"

[lrhn]

I'd be fine with ! being a constant expression. However, I didn't find it in the "Constant Objects" section of the NNBD specification. Was it added later @eernstg?

(I did find a section saying that the section on as checks being covariant should be changed to'

\item An expression of the form \code{$e$\,\,as\,\,$T$} or \code{$e$\,\,is\,\,$T$} is potentially constant if $e$ is a potentially constant expression and $T$ is a potentially constant type expression, and it is further constant if $e$ is constant.

That should also say "and $T$ is constant" at the end.)

[eernstg]

Same here, I can't see any justification for the claim that a constant expression can contain a selector of the form !, neither in the language specification, nor in the NNBD feature specification, nor in any other specification document that I could come up with. ! is only used as a boolean negation operator, in !=, and in is!.

@sgrekhov, did you find anything to the contrary somewhere?

This means that the CFE should be modified to report an error.

[eernstg]

@keertip, I adjusted the labels. PTAL if you disagree.

[eernstg]

We could consider generalizing constant expressions to include this case, but that would be a fresh language repo issue, so I won't proceed with anything of that nature here.