class A<X> {
  const A(x) : x = x is List<X> ? x : x as List<X>;
  final List<X> x;
}

class B<X> {
  const B() : f = m;
  final void Function(X) f;
}

class C<X> {
  const C() : f = m<X>;
  final f;
}

void m<X>(X x) {}
void main() {}