No error in CFE in case of generic function override

[sgrekhov]

I searched but didn’t find whether a similar issue already exists. Please close this one as a duplicate if it does.

The code below produces an error in the analyzer but works in CFE.

class A<T extends num> {
  X foo<X extends T>(X x) => x;
}

class C<T extends num> extends A<num> {
  int v;
  C(this.v);
  X foo<X extends T>(X x) => x + v as X; // Analyzer error.  'C.foo' ('X Function<X extends T>(X)') isn't a valid override of 'A.foo' ('X Function<X extends num>(X)').
}

void main() {
  var c = C<int>(1);
  print(c.foo(1)); // prints 2
}

@eernstg which tool is right here?

Dart SDK version: 3.8.0-38.0.dev (dev) (Sat Jan 25 16:01:44 2025 -0800) on "windows_x64"

[dart-github-bot]

Summary: Analyzer incorrectly flags foo override in subclass C as invalid. The code compiles and runs correctly in CFE.

[eernstg]

Good catch!

The analyzer behavior is correct: The method signature X foo<X extends T>(X) in C<T> is not a correct override of X foo<X extends num>(X) in the superinterface A<num> of C<T>, because the function type X Function<X extends T>(X) isn't a subtype of X Function<X extends num>(X).

The basic reason for this failure is that a type parameter bound in a generic function type is an invariant position in the type, and there is no knowledge about X and num that would prove that they are the same type (that is: mutual subtypes).