This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2023-01-15


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: TC1     Submitter: Bill Gibbons     Date: 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,
9.3.4.7 [dcl.fct.default] paragraph 9 says:
When a declaration of a function is introduced by way of a using-declaration (9.9 [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-declarations — 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]