Analyzer const cast fails for Never?.

[lrhn]

Example:

  const v = null as Never?;

The analyzer reports a compile-time error for this:

line 3 • Evaluation of this constant expression throws an exception.

Doesn't say which exception, and it shouldn't since null is a valid value of any nullable type.
(Doesn't check type for nullability?)

[bwilkerson]

I'm guessing that Never? means that it will either return null or never return. There are likely other places in the analyzer / linter where Never? isn't being handled correctly. For example, the code

Never? f() => null;

results in the lint Unnecessary use of the type 'Null'. Try using 'void' instead.. This seems like a poor message, at the very least.

@scheglov @pq

[pq]

I'm inclined to agree that at least prefer_void_to_null is overreporting.

Issue filed: #59388

[eernstg]

We've got this one as well: #59309.

[pq]

Yes, thanks @eernstg!

[lrhn]

I'm guessing that Never? means that it will either return null or never return.

Or throw, just like Null.
The type Never? is NORM-equivalent to Null, which means that it should behave exactly the same in most contexts (likely all contexts for this type, a few other types have a few exceptions where the pre-normalization type matters).

It's definitely true that Never? <: Null and Null <: Never?, and their Type objects are equal, and identical if const:

typedef NeverQ = Never?;
void main() { 
  assert(Null == NeverQ);
  const t1 = Null; 
  const t2 = NeverQ;
  assert(identical(t1, t2));
}

[sgrekhov]

There is definitely wrong error message in the analyzer in case of type Never?. And no compile error in CFE. What should we expect in the case below?

class C {
  Never? get foo => throw "Exception";
}

main() {
  C c = C();
  try {
    c.foo?.bar(); // CFE: No compile error. Analyzer: The method 'bar' can't be unconditionally invoked because 
                  // the receiver can be 'null'. Try making the call conditional (using '?.') or adding a null 
                  // check to the target ('!'). 
  } catch (_) {}
}

Dart SDK version: 3.9.0-43.0.dev (dev) (Tue Apr 22 13:03:05 2025 -0700) on "windows_x64"