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(foo());
}

In function 'int main()':
Line 16: error: invalid initialization of non-const reference of type 'foo&' from a temporary of type 'foo'
compilation terminated due to -Wfatal-errors.