Analyzer: report an error when `Future<nn-type>.value()` or `Completer<nn-type>.complete()` called with nullable value

[srawlins]

In #45319 @simolus3 introduced an analyzer Warning that reported when a null value is (implicitly or explicitly) given as an argument to Future.value() or Completer.complete() and the type argument on the Future constructor or Completer instance is non-nullable.

At the end of the conversation he wrote:

I can change it to handle "nullable argument, non-nullable type" too but I'm not sure if it should be a default diagnostic if it can have false positives? In particular, it would warn about Future.value(x as dynamic) then.

I think your decision is sound. Later on, we can trigger on nullable arguments either in the same Hint, or with a new Hint (with a different message).

Thinking about this again, 2 years later, I think we should still report the "nullable argument" case (like int? x; Future<int>.value(x);), and I think we should report it as a default enabled Warning. With the following semantics, I think there are no false positives that diverge from the standard assignability rules, acting as if the parameter was typed FutureOr<T> rather than FutureOr<T>?:

• Given a call to Future<T>.value (constructor) or Completer<T>.complete (method), where T is non-nullable:
  • If the argument type is dynamic, do not report anything.
  • If the argument type is potentially nullable, report a Warning.