Dart does not throw error if function argument parameter extends Null

[iarkh]

Dart SDK Version: 2.0.0-dev.55.0
OS: Windows 10

Sample code below declares a [typedef] with parameter which extends [Null]. It causes a lot of compiler errors with dartanalyzer and passes without errors with dart.

typedef F<X extends Null> = Function(X);
main() {}

This is similar to the issue #33699, however analyzer produces different errors in these two cases, so we would be better to have separate issues.

I believe both tools should behave in the same way.
Dartanalyzer sample output is:

Analyzing typedef_ret_extends_neg_l1_t05.dart...
  error - 'X' can't be a supertype of its upper bound at typedef_ret_extends_neg_l1_t05.dart:50:11 - type_parameter_supertype_of_its_bound
1 error found.

Dart stdout and stderr are empty, no exceptions or warnings there.

[lrhn]

The error seems spurious.

There is nothing inherently problematic with allowing Null as a type parameter bound (apart from the class being useless, so it's also not a big loss if we don't).

If we have a rule preventing a type parameter from being a supertype of its bound, the phrasing needs to be fixed so it doesn't allow this any more than it disallows <X extends Object>.

[iarkh]

OK, but if Null is allowed, analyzer should not throw error too.
Should I update synopsys accordingly?

[eernstg]

We could simply accept the current wording including the error with an explicitly Null bound, noting that it is really not a useful case anyway.

In order to ensure that this issue can stay focused on its original purpose, I've created a new issue #33709 addressing the language question. Marking this issue as blocked: Waiting for #33709.

[eernstg]

Removing blocked status: As of 4fd0404, the subtype circularity prevention rule in the language specification has be adjusted such that Null is now an acceptable upper bound for a type variable. The front end does not report an error, which is ok, but the analyzer should now be updated to accept the example program.

So we need to change the area back to analyzer—done.

[srawlins]

Analyzer no longer reports an error for this code (both with and without null-safety).