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Example in [basic.lookup.argdep]/3:

namespace NS {
    class T { };
    void f(T);
    void g(T, int);
}
NS::T parm;
void g(NS::T, float);
int main() {
    f(parm); // OK: calls NS::f
    extern void g(NS::T, float);
    g(parm, 1); // OK: calls g(NS::T, float)
}

For the call g(parm, 1) we have in the set X the declaration void g(NS::T, float); in global scope. AFAICT, we also have in set Y the declaration void g(T, int); in namespace NS, an associate namespace of the argument parm of type NS::T. Thus, if I'm not wrong, the two declarations are candidates for overload resolution. Then, is it the case that the declaration in global scope is preferred in relation to the one in namespace NS? Why? I would very much appreciate a quote from the Standard as an answer.

curiousguy
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WaldB
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  • Unqualified name lookup finds a viable candidate, and so lookup stopped. – Passer By Aug 03 '18 at 21:42
  • @PasserBy Well [basic.lookup.argdep]/3 says explicitly `The set of declarations found by the lookup of the name is the union of X and Y.` – WaldB Aug 03 '18 at 21:49
  • *"the lookup"* refers to ADL. – Passer By Aug 03 '18 at 21:50
  • @PasserBy And that's exactly what I'm talking about. See this in [\[basic.lookup.unqual\]/3](http://eel.is/c++draft/basic.lookup#unqual-3): `The lookup for an unqualified name used as the postfix-expression of a function call is described in [basic.lookup.argdep]`. – WaldB Aug 03 '18 at 21:55

1 Answers1

2

In that section, we have:

Let X be the lookup set produced by unqualified lookup and let Y be the lookup set produced by argument dependent lookup (defined as follows). If X contains

  • a declaration of a class member, or
  • a block-scope function declaration that is not a using-declaration, or
  • a declaration that is neither a function nor a function template

then Y is empty.

Unqualified lookup finds the block scope function declaration - extern void g(NS::T, float). Not the declaration at global scope ::g.

Hence, Y is empty, which makes the union of X and Y trivially just X, which just contains that one declaration, which is a viable candidate.

If that declaration were not there, then unqualified lookup would find void g(NS::T, float) and we'd go on to perform ADL, which would find N::g(T, int), which is a better match.

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Barry
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