From: "Tobias Schwinger" <tschwinger@neoscientists.org>
My apologies - please, just forget what I've posted.
no_overload_tag function( \ BOOST_PP_ENUM_PARAMS(BOOST_PP_DEC(arity) \ , any BOOST_PP_INTERCEPT)...); \
GCC allows this for binary operators (even in ansi mode), making me jump to conclusions - couldn't find anything in the standard backing it.
Further I doubt there is a chance of weakening the "no-match-overload" in any other way to match worse than a function with one user defined conversion per argument.
Actually, there is...: --- Start --- #include <iostream> #include <boost/config.hpp> // For BOOST_STATIC_CONSTANT struct proxy { proxy(...); }; struct no_match_type {}; char op_test(...); char (&op_test(const no_match_type &))[2]; no_match_type operator+(const proxy &,const proxy &); template<class T,class U> struct has_plus_op { static T t; static U u; typedef has_plus_op type; BOOST_STATIC_CONSTANT(bool, value=sizeof(op_test(t+u))==1); }; class A { public: A(int); // Note user-defined conversion }; class B {}; A operator+(A,A); int main() { std::cout << has_plus_op<int,int>::value << "\n"; // Sure std::cout << has_plus_op<int,double>::value << "\n"; // No sweat (standard conversion - promotion) std::cout << has_plus_op<int,A>::value << "\n"; // Again, no problem UDC int -> A takes preference to ... std::cout << has_plus_op<A,B>::value << "\n"; // Nope } --- End --- This shows that using "proxy" allows implicit promotions/conversions, as well as user-defined conversions, to take place as part of the overload resolution. You can't use "..." as an operator parameter (as you found out), but you _can_ get around it like this. ;) The above is the "modified" isAddable that I mentioned in the OP, and which is found (with appropriate namespace protection) in operator_traits/detail/binop.hpp. It's good to be back. ;) Regards, Terje