Least and greatest closures works incorrectly for invariant generic typedef: unexpected compile error appears

[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(X x);

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

Seems like it should print (Object?) => Object? twice here, however this does not happen and dart throws a compile error instead.

Dart sample output is:

test.dart:22:11: Error: The argument type 'Object? Function(Never) Function()' can't be assigned to the parameter type 'Object? Function(Object?) Function()'.

• 'Object' is from 'dart:core'.
  try { f(() => captureTypeArgument()); } catch(_) {};
  ^

Related issues against covariant and contravariant cases are: #44161, #44162.

[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(dynamic). No errors at compile-time here.

capturedTypeArgument is the greatest closure of _ Function(_) with respect to {_}, which is Object? Function(Never). Note that there is no type T such that T Function(T) is this type, or even a supertype of this type, and this means that it is not possible to choose a type argument T to f such that an actual argument of type Object? Function(Never) Function() is type correct, and hence we get the compile-time error as described.