[code]
class A {
  int foo() { return 1; }
}

class B {
  this(int a) {}
}

void main() {
  A a = new A();
  B b = new B(a.foo); // error 'constructor this (int a) does not match argument types (int())'
}
[/code]

If replace a.foo with a.foo(), all works.