17 Templates [temp]

17.6 Name resolution [temp.res]

17.6.4 Dependent name resolution [temp.dep.res] Candidate functions [temp.dep.candidate]

For a function call where the postfix-expression is a dependent name, the candidate functions are found using the usual lookup rules ([basic.lookup.unqual], [basic.lookup.argdep]) except that:

If the call would be ill-formed or would find a better match had the lookup within the associated namespaces considered all the function declarations with external linkage introduced in those namespaces in all translation units, not just considering those declarations found in the template definition and template instantiation contexts, then the program has undefined behavior.