r = engine->RegisterObjectMethod("string", "bool opEquals(const string &in) const", asMETHODPR(sf::String, operator ==, (const sf::String &) const, bool), asCALL_THISCALL); assert( r >= 0 );
Hopefully it should work. If not, please post what the SFML string class looks like.
The string type is really just an ordinary type in AngelScript, and is registered just like that with the only exception being the additional string factory. I include registration for the std::string because it is the most common, but many developers use their own implementation.
std::basic_string<Uint32> is a completely different type than std::basic_string<char>, a.k.a std::string, so the fact that both are implemented from the same template doesn't really make things any easier here.