On Tue, Jan 10, 2023 at 11:24 AM Edward Catmur via Std-Discussion <
std-discussion_at_[hidden]> wrote:

>
>
> On Tue, 10 Jan 2023 at 15:57, language.lawyer--- via Std-Discussion <
> std-discussion_at_[hidden]> wrote:
>
>> > I would like an example of a function call to an overloaded function,
>> > where the most viable function is determined by the rules listed from
>> > [over.ics.rank]/4.4.5 to [over.ics.rank]/4.4.8:
>> >
>> > https://timsong-cpp.github.io/cppwp/n4868/over.ics.rank#4.4.5
>> >
>> > These are four different tie-breakers for ranking standard conversions
>> > from different source types, assuming the following class hierarchy:
>> >
>> > struct A {}; struct B : C {}; struct C : B {};
>> >
>> > (4.4.5) conversion of B* to A* is better than conversion of C* to A*,
>> > (4.4.6) binding of an expression of type B to a reference to type A is
>> > better than binding an expression of type C to a reference to type A,
>> > (4.4.7) conversion of B::* to C::* is better than conversion of
>> > A::* to C::*, and
>> > (4.4.8) conversion of B to A is better than conversion of C to A.
>> >
>> > The note says that these are only used for tie-breakers in the second
>> > conversion sequence of user-defined conversions:
>> >
>> > https://timsong-cpp.github.io/cppwp/n4868/over.ics.rank#note-1
>>
>> I think it is just a wrong Note.
>> CD2 (November 96
>> https://www.open-std.org/jtc1/sc22/wg21/docs/wp/html/cd2/over.html#over.ics.rank)
>> Note was saying:
>> > [Note: it is necessary to compare conversions with different target
>> > types in the context of an initialization by user-defined conver-
>> > sion; see _over.match.best_. ]
>>
>> Which also seems not 100% correct (incomplete?), because it mentions
>> (only?) target, and not source types.
>>
>> The next WP (Oct'97
>> https://www.open-std.org/jtc1/sc22/wg21/docs/wp/html/oct97/over.html#over.ics.rank)
>> Note gained its current (defective) wording.
>>
>> (The bullets themselves have been added by
>> https://www.open-std.org/JTC1/sc22/wg21/docs/papers/1995/N0661.asc)
>>
>> So, ecatmur's example seems to be relevant here, except that the Note is
>> not about the second standard conversion sequence after different user
>> conversion functions, but about
>> https://timsong-cpp.github.io/cppwp/n4868/over.match.best#general-2.2
>>
>
> Ah. So, currently, the Note says:
>
> > Compared conversion sequences will have different source types only in
> the context of comparing the second standard conversion sequence of an
> initialization by user-defined conversion (see [over.match.best]); in all
> other contexts, the source types will be the same and the target types will
> be different.
>
> Whereas [over.match.best.general]/2.2 says:
>
> > ... the context is an initialization by user-defined conversion (see
> [dcl.init], [over.match.conv], and [over.match.ref]) and the standard
> conversion sequence from the return type of F1 to the destination type
> (i.e., the type of the entity being initialized) is a better conversion
> sequence than the standard conversion sequence from the return type of F2
> to the destination type ...
>
> Would it make sense to amend the latter to mention the second SCS? i.e.,
>
> > ... the context is an initialization by user-defined conversion (see
> [dcl.init], [over.match.conv], and [over.match.ref]) and the
> <ins>second</ins> standard conversion sequence <ins>of ICSj(F1) (i.e.,
> that</ins> from the return type of F1 to the <del>destination type
> (i.e.,</del> the type of the entity being initialized) is a better
> conversion sequence than the <ins>second</ins> standard conversion sequence
> <del>from the return type of F2 to the destination type</del><ins>of
> ICSj(F2)</ins> ...
>

I think if we need to clarify it, we should just add a note. Changing
normative wording always risks unforeseen consequences.


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-- 
*Brian Bi*