The syntax for declaring const member functions is not very clear.  There is no grammar.  You need to infer from various examples what to do.  For instance:

class B {}
class A {
  B x;
  const B getB() { return B; }
}

doesn't compile, because this is a const function returning a nonconst value.   I had to find the answer on a forum.  It's challenging to figure this out from the docs:

class B {}
class A {
  B x;
  const const(B) getB() { return B; }
}

Examples covering more use cases would probably help, as well as more formally describing how const is specified.  Perhaps modifying the example of an invariant member function to return a value (other than string) would help.