[analyzer] Function.call does not get the type Function

[eernstg]

Consider the following program:

typedef Exactly<X> = X Function(X);

extension StaticTypeHelper<X> on X {
  X expectStaticType<Y extends Exactly<X>>() => this;
}

void main() {
  Function d = print;
  var f = d.call;
  f.expectStaticType<Exactly<Function>>(); // No error (analyzer and CFE agree).
  int i = f; // No error according to the analyzer.
}

The CFE reports a compile-time error on the initializing expression of the local variable i, but the analyzer accepts the declaration of i with no diagnostic messages (seen in DartPad based on Dart SDK 3.3.0-273.0.dev).

When the insertion point in DartPad is at the declaration of f, the Documentation pane indicates that the type of f is InvalidType (which is probably the reason why no further errors are reported, no matter how f is used).

However, the static type of f should have been Function: https://github.com/dart-lang/language/blob/daf5cbd39e5b23bf7836dfa0926d0cfe4ae663b9/specification/dartLangSpec.tex#L14799.

[eernstg]

The soundness issue comes up because there is no compile-time error in the given example program according to the analyzer, but if we actually allow such a program to run (that is, if the CFE also hadn't reported any errors) then we would create a situation where a variable of type int refers to an object of type void Function(Object?), which is a soundness violation.

[bwilkerson]

@scheglov

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/345364