Variable not promoted to Null after null-check

[julemand101]

Tested against the following Dart versions:

Dart SDK version: 2.16.1 (stable) (Tue Feb 8 12:02:33 2022 +0100) on "windows_x64"
Dart SDK version: 2.17.0-150.0.dev (dev) (Fri Feb 25 22:29:59 2022 -0800) on "windows_x64"

The following program shows some "interesting" behavior when it comes to promote a variable to Null when doing a null-check:

extension NullExtension on Null {
  void someMethod() {
    print("Called Null.someMethod");
  }
}

String? getString() => null;

void main() {
  final string = getString();

  if (string == null) {
    //(string as Null).someMethod();
    string.someMethod();
  }
}

This fails by the analyzer with:

The method 'someMethod' can't be unconditionally invoked because the receiver can be 'null'. (Documentation)
Try making the call conditional (using '?.') or adding a null check to the target ('!').

And when running the program:

bin/test.dart:14:12: Error: The method 'someMethod' isn't defined for the class 'String?'.
Try correcting the name to the name of an existing method, or defining a method named 'someMethod'.
    string.someMethod();
           ^^^^^^^^^^

If I uncomment (string as Null).someMethod(); as:

extension NullExtension on Null {
  void someMethod() {
    print("Called Null.someMethod");
  }
}

String? getString() => null;

void main() {
  final string = getString();

  if (string == null) {
    (string as Null).someMethod();
    string.someMethod();
  }
}

The program does no longer have any warning or error from the analyzer and runs with the following result:

Called Null.someMethod
Called Null.someMethod

This behavior seems kinda weird since the promotion to Null seems to happen based on a local type-cast that should not affect the rest of the program.

This might also explain why auto-completion is not that smart (tested with IntelliJ) when it comes to showing what methods can be called on the value. If I use auto-complete before (string as Null).someMethod(); I will get the full list of methods as if the string variable was a String:

While if I do the same after (string as Null).someMethod(); I will get the possible methods for Null:

[eernstg]

We do have the following in the null safety spec:

A check of the form e == null or of the form e is Null where e has static
type T promotes the type of e to Null in the true continuation, and to
NonNull(T) in the
false continuation.

I think we've discussed not performing this particular promotion (because of the rather low value of having an expression of type Null), but I can't find any evidence that we shouldn't promote.

@scheglov, @johnniwinther, WDYT?

[johnniwinther]

I can recall some late changes to how we handle null in the promotion (something to do with unsound mode) but can't recall the details. @stereotype441 might remember more since he did most of the implementation.

[stereotype441]

I don't recall whether there was a deliberate decision at the time we implemented it, but sometime later, we did consider adding this functionality, in response to dart-lang/language#1505. I did some analysis back in November, and IMHO, enough code would be broken by the change that it's not worth it (you can see my analysis here: dart-lang/language#1505 (comment)). But the question hasn't risen to a high enough priority for the language team to discuss it and make a firm yes/no decision, so the issue remains open.

(@johnniwinther I think you may have been thinking of dart-lang/language#1143, which is a different issue).

I'm going to close this issue on the grounds that it's a duplicate of dart-lang/language#1505. If folks want to discuss it further, let's have the discussion there so that all the information is in one place.