class foo {
  friend int lvalue_only(foo &);
  int f() const {
    return 1;
  }
};

int lvalue_only(foo & x) {
  return x.f();
}

int main() {
  foo x;
  lvalue_only(x);
  lvalue_only(static_cast<foo&>(foo()));
}

In function 'int main()':
Line 15: error: invalid static_cast from type 'foo' to type 'foo&'
compilation terminated due to -Wfatal-errors.