analyzer does not report an error on incorrect code in strong mode

[mraleph]

typedef T RecognizerCallback<T>();

class Test {
  T invokeCallback<T>(RecognizerCallback<T> callback) {
    return callback();
  }

  void callback() {
  }

  void test() {
    invokeCallback<String>(callback);
    invokeCallback<Null>(callback);
  }
}

╭─ ~/src/dart/sdk ‹master›
╰─$ sdk/bin/dartanalyzer --strong -v t/analysis.dart                                                                                3 ↵
Analyzing t/analysis.dart...
Using default analysis options
No issues found!

If I now move the code to top-level scope then analyzer reports an error

typedef T RecognizerCallback<T>();

  T invokeCallback<T>(RecognizerCallback<T> callback) {
    return callback();
  }

  void callback() {
  }

  void test() {
    invokeCallback<String>(callback);
    invokeCallback<Null>(callback);
  }

╭─ ~/src/dart/sdk ‹master›
╰─$ sdk/bin/dartanalyzer --strong -v t/analysis.dart
Analyzing t/analysis.dart...
Using default analysis options
  error • The function 'callback' has type '() → void' that isn't of expected type '() → String'. This means its parameter or return type does not match what is expected at t/analysis.dart:13:28 • strong_mode_invalid_cast_function
  error • The function 'callback' has type '() → void' that isn't of expected type '() → Null'. This means its parameter or return type does not match what is expected at t/analysis.dart:14:26 • strong_mode_invalid_cast_function
1 error found.

[Flutter is full of this sort of mistyped code (except they use Null instead of String) , so once this is fixed we would heed to do a pass over their code and fix it /cc @leafpetersen ]

@devoncarew could you help triage this? Thanks.

[vsmenon]

I'm guessing that is a bug with instance-method (e.g., callback here) handling.

[vsmenon]

Commenting out the isStatic check here:

https://github.com/dart-lang/sdk/blob/master/pkg/analyzer/lib/src/task/strong/checker.dart#L1214

does cause the error to be reported in Slava's first example. Not sure that is quite the right fix though.

[peter-ahe-google]

@mraleph has a workaround: Fasta produces the errors he actually need to fix.

[stereotype441]

I added this to our analyzer tracking bug (#31638) so we won't forget to work on it

[leafpetersen]

The strong mode semantics of that for both cases is an implicit downcast. That implicit downcast will in this case fail at runtime, but it is only in the toplevel case that the analyzer can prove that the downcast will definitely fail at runtime. In those cases, strong mode turns the downcast into a static error.

[vsmenon]

So the question is if Slava's original example should a static error or a runtime error?

[mraleph]

@leafpetersen ah, implicit down casts. of course - () -> void is a super type of any () -> T.

FWIW this is extremely confusing and there are >30 incorrectly typed call-sites in Flutter due to this. Fixing this one call-site at a time would take us quite a bit of time.