Least and greatest closures are incorrect for covariant generic typedef check<X> = X Function()

[iarkh]

Dart SDK version: 2.12.0-29.0.dev (dev) (Mon Nov 9 06:42:00 2020 -0800) on "windows_x64"

Here is a source code example:

Type? _capturedTypeArgument;

X captureTypeArgument<X>() {
  _capturedTypeArgument = X;
  throw "Error";
}

Type? get capturedTypeArgument {
  var result = _capturedTypeArgument;
  _capturedTypeArgument = null;
  return result;
}

typedef F<X> = void Function<Y extends X>();
F<X> toF<X>(X x) => <Y extends X>() {};
Type typeOf<X>() => X;

typedef check<X> = X Function();

void main() {
  void f<X>(check<X> Function() g) => g();
  try { f(() => captureTypeArgument()); } catch(_) {};
  print (typeOf<check>());
  print(capturedTypeArgument);
}

Seems like Object? should be substituted as a bound both for check and capturedTypeArgument here, however this is not so and sample output is:

() => dynamic
() => Object?

Please note that for another covariant case - typedef check<X> = X Function() - greatest closure is Object? in both cases.

See also similar issue against contravariant case: #44161.

[eernstg]

Cf. #44161 for a bit more detail.

This is working as intended: typeOf<check>() is obtained by applying i2b on the raw type check, and the resulting type is dynamic Function(). capturedTypeArgument is the greatest closure of _ Function() with respect to {_}, which is Object? Function().