Least and greatest closures are incorrect for contravariant generic typedef

[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> = void Function(X x);

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

Seems like Never should be substituted as a bound for the check and capturedTypeArgument here, however this is not so and sample output is:

(dynamic) => void
(Never) => void

[eernstg]

This is actually working as intended:

print(typeOf<check>()) should print something that means void Function(dynamic), because the meaning of the raw type check is check<dynamic> (using i2b), and that matches the reported printout (dynamic) => void.

print(capturedTypeArgument) should print something that means void Function(Never), which is again a match for (Never) => void. The latter is the greatest closure of void Function(_) with respect to {_} (the set that just contains _), as it should be.

We did have discussions about the initial values for type variables used in the i2b algorithm, and we did consider using the bottom type as the initial value corresponding to a contravariant type parameter, but we ended up using the declared bound with dynamic as the default value.

So I believe we can close this issue.