argument-dependent name lookup

Argument-dependent name lookup applies to the lookup of an unqualified function name depending on the types of the arguments given to the function call. 
During argument-dependent lookup, other namespaces not considered during normal lookup may be searched where the set of namespaces to be searched depends on the types of the function arguments. Specifically, the set of declarations discovered during the process, and considered for resolution of the function name, is the union of the declarations found by normal lookup with the declarations found by looking in the set of namespaces associated with the types of the function arguments.
```
namespace NS {

class A {};
void f(A& a) {}
}  // namespace NS

int main() {
   NS::A a;
   f(a);  // Calls NS::f.
}
```
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