This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-12-19
[Moved to DR at the February, 2014 meeting as part of document N3914.]
Currently, 12.2.4.2 [over.best.ics] paragraph 4 reads,
However, when considering the argument of a constructor or user-defined conversion function that is a candidate by 12.2.2.4 [over.match.ctor] when invoked for the copying/moving of the temporary in the second step of a class copy-initialization, by 12.2.2.8 [over.match.list] when passing the initializer list as a single argument or when the initializer list has exactly one element and a conversion to some class X or reference to (possibly cv-qualified) X is considered for the first parameter of a constructor of X, or by 12.2.2.5 [over.match.copy], 12.2.2.6 [over.match.conv], or 12.2.2.7 [over.match.ref] in all cases, only standard conversion sequences and ellipsis conversion sequences are considered.
This is cumbersome and hard to understand. A possible improvement might be:
However, only standard conversion sequences and ellipsis conversion sequences are considered if:
the parameter is the first parameter of a constructor of a class X, or
the parameter is the implicit object parameter of a user-defined conversion function, and
the constructor or user-defined conversion function is a candidate by:
12.2.2.4 [over.match.ctor] — when the argument is the temporary being copied/moved in the second step of a class copy-initialization.
12.2.2.5 [over.match.copy], 12.2.2.6 [over.match.conv], or 12.2.2.7 [over.match.ref] — in all cases.
12.2.2.8 [over.match.list] — during phase two, when the argument was the only element in the initializer list, and the parameter is of type X or reference to (possibly cv-qualified) X.
(Note that this rewording removes the restriction that applies during phase one of 12.2.2.8 [over.match.list], as there is no longer any way to trigger it due to the fact that only initializer-list constructors are candidates. See this bug report for details.)
Proposed resolution (September, 2013) [SUPERSEDED]:
Change 12.2.4.2 [over.best.ics] paragraph 4 as follows:
However,
when considering the argument of a constructor or user-defined conversion function that is a candidate by 12.2.2.4 [over.match.ctor] when invoked for the copying/moving of the temporary in the second step of a class copy-initialization, by 12.2.2.8 [over.match.list] when passing the initializer list as a single argument or when the initializer list has exactly one element and a conversion to some class X or reference to (possibly cv-qualified) X is considered for the first parameter of a constructor of X, or by 12.2.2.5 [over.match.copy], 12.2.2.6 [over.match.conv], or 12.2.2.7 [over.match.ref] in all cases, only standard conversion sequences and ellipsis conversion sequences are considered.if the target is
the first parameter of a constructor of a class X or
the implicit object parameter of a user-defined conversion function,
and the constructor or user-defined conversion function is a candidate by
12.2.2.4 [over.match.ctor], when the argument is the temporary acting as the source in the second step of a class copy-initialization,
12.2.2.5 [over.match.copy], 12.2.2.6 [over.match.conv], or 12.2.2.7 [over.match.ref] (in all cases), or
the second phase of 12.2.2.8 [over.match.list] when the initializer list has exactly one element, and the conversion is to X or reference to (possibly cv-qualified) X,
user-defined conversion sequences are not considered. [Example:
struct X { X(); }; struct B { operator X&(); }; B b; X x({b}); // error: B::operator X&() is not a candidate—end example]
Additional note (October, 2013):
Questions have been raised about several of the bullets in the September, 2013 proposed resolution and whether a note would be preferable instead of or in addition to the example . The issue has been returned to "review" status to allow consideration of these questions.
Additional note (January, 2014):
It has also been observed that the proposed resolution would make the following example ill-formed by preventing the consideration of B's conversion function when initializing the first parameter of A's copy constructor:
struct A {
A() {}
A(const A &) {}
};
struct B {
operator A() { return A(); }
} b;
A a{b};