This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-10-26
[Moved to DR at the October, 2015 meeting.]
In an example like
struct A { operator int(); };
template<typename T> T operator<<(T, int);
void f(A a) { 1 << a; }
Template argument deduction succeeds for the operator template, producing the signature operator<<(int,int). The resulting declaration is synthesized and added to the overload set, per 13.10.4 [temp.over] paragraph 1. However, this violates the requirement of 12.4 [over.oper] paragraph 6,
An operator function shall either be a non-static member function or be a non-member function that has at least one parameter whose type is a class, a reference to a class, an enumeration, or a reference to an enumeration.
This is not a SFINAE context, so the program is ill-formed, rather than selecting the built-in operator.
Proposed resolution (May, 2015):
Change 13.10.4 [temp.over] paragraph 1 as follows,
...If, for a given function template, argument deduction fails or the synthesized function template specialization would be ill-formed, no such function is added to the set of candidate functions for that template.