CWG Issue 1 (original) (raw)

This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 118e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-11-05

1. What if two using-declarations refer to the same function but the declarations introduce different default-arguments?

Section: 9.3.4.7 [dcl.fct.default]Status: TC1Submitter: Bill GibbonsDate: unknown

6.4 [basic.scope] paragraph 4 says:

Given a set of declarations in a single declarative region, each of which specifies the same unqualified name,

• they shall all refer to the same entity, or all refer to functions ...

9.3.4.7 [dcl.fct.default] paragraph 9 says:

When a declaration of a function is introduced by way of a_using-declaration_ (9.10 [namespace.udecl]), any default argument information associated with the declaration is imported as well.

This is not really clear regarding what happens in the following case:

namespace A {
        extern "C" void f(int = 5);
}
namespace B {
        extern "C" void f(int = 7);
}

using A::f;
using B::f;

f(); // ???

Proposed resolution (10/00):

Add the following at the end of 12.2.4 [over.match.best]:

If the best viable function resolves to a function for which multiple declarations were found, and if at least two of these declarations — or the declarations they refer to in the case of_using-declaration_s — specify a default argument that made the function viable, the program is ill-formed. [Example:
namespace A {
extern "C" void f(int = 5);
}
namespace B {
extern "C" void f(int = 5);
}

using A::f;
using B::f;

void use() {
f(3); // OK, default argument was not used for viability
f(); // Error: found default argument twice
}

—_end example_]