Quick A: The signature of the function is first.

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Syntax of final, override, const with trailing return types

The correct syntax should be:

• override and final should appear after the member function declaration, which including the trailing return type specification, i.e.

auto debug(ostream& os=cout) const ->ostream& override final;

• override and final should not be used with the member function definition outside the class definition, so just remove them:

auto Derived::debug(ostream& os) const ->ostream&
{
  os << "dval: " << dval << endl;
  return os;
}