Analyzer should not accept a typedef that refers to itself via bound

[stereotype441]

The following program is accepted by the analyzer but rejected by the CFE:

typedef F<T extends F<Null>> = T Function();

main() {}

The CFE's error message is: Error: The typedef 'F' has a reference to itself.

I believe the analyzer should reject this program, because of the following spec text (from the "Type Aliases" section):

Any self reference in a type alias, either directly or recursively via another type declaration, is a compile-time error.

We will probably want to fix this issue either before or during our work on analyzer support for non-function type aliases.

[stereotype441]

After further experimentation it seems that the analyzer and the CFE have a number of differences in how they check for circularities. I've filed a language issue, and I'll postpone further work on this until that language issue is resolved: dart-lang/language#1261

[leafpetersen]

@eernstg do we have a resolution for this?

[eernstg]

I've proposed an update to the language specification in dart-lang/language#1468 which specifies the strict approach (where both body dependencies and bound dependencies give rise to a cyclicity error). This allows us to relax the rule later on (to allow bound dependencies), which would be a non-breaking change.

I've added a language test in https://dart-review.googlesource.com/c/sdk/+/186290, covering areas that weren't already covered for this topic.

The example here is flagged as an error by both the analyzer and the CFE today.

The new test causes a crash in the CFE, cf. #45085, but the analyzer already passes the new test (and the existing ones), so there is no implementation effort associated with the language rule itself.

[leafpetersen]

This seems to be fixed.