[extension types/analyzer] The receiver can't be null false negative on extension type typed values?

[modulovalue]

Consider:

void main() {
  const Foo a = Foo(0);
  const int b = 0;
 
  a?.toString();
// The receiver can't be null, so the null-aware operator '?.' is unnecessary.
// v
  b?.toString();
}

extension type const Foo(int i) {}

b?.toString(); causes the analyzer to emit a The receiver can't be null, so the null-aware operator '?.' is unnecessary. diagnostic, but it looks to me like a?.toString(); could also be annotated with a similar diagnostic.

[dart-github-bot]

Summary: The analyzer incorrectly fails to detect that a null-aware operator is unnecessary for an extension type (Foo) when it should, as it does for a regular type (int). This suggests a potential false negative in the analyzer's null-aware operator analysis for extension types.

[lrhn]

It's reasonable. The type of b is int which is a subtype of Object. That guarantees that the value cannot be null.

The type of a is Foo, which is not a subtype of Object, and also not a supertype of Null.

The current representation type of Foo is int, which means that the actual value won't be null, but depending on design, it may be reasonable and non-breaking for the author to change the representation type in a future version, maybe to int?.
The user of the type Foo is not allowed to assume that the value cannot be null, because that would make it breaking for the author to change that.