Dart Null Safety Promotion

null_safety_promotion.dart

main([args]) {
  String? x = args == null ? 'foo' : null;
  {
    String? y = x;
    ignore(y!); // Promotes along non-throwing branch;
    print(y.length);
  }

  {
    String? y = x;
    ignore(y as String); // Promotes along non-throwing branch;
    print(y.length);
  }

  {
    String? y = x;
    // Promotes along true branch.
    if (y != null) print(y.length);
  }

  {
    String? y = x;
    // Promotes along false branch.
    if (y == null) {
    } else {
      print(y.length);
    }
  }

  {
    String? y = x;
    // Promotes along true branch.
    if (y is String) print(y.length);
  }

  {
    String? y = x;
    // Promotes along true branch, but recommends `y != null`.
    if (y is! Null) print(y.length);
  }

  // DOES NOT PROMOTE
  {
    String? y = x;
    // Does not promote even though y is String? && y is Object => y is String
    if (y is Object) print(y.length);
    // Does not promote even though `identical(y, null)` => `y == null`
    if (!identical(y, null)) print(y.length);
    if (identical(y, null)) {
    } else {
      print(y.length);
    }
    // Does not promote even though `y == null` => `y?.length == null`
    ignore((y?.length)!);
    print(y.length);
    // Does not promote even though `y == "foo"` => `y != null`.
    if (y == "foo") print(y.length);
  }
}


void ignore(_) {}