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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/102283] Inconsistent/wrong overload resolution when using an initializer list and a defaulted template parameter Date: Thu, 16 Sep 2021 13:40:57 +0000 [thread overview] Message-ID: <bug-102283-4-TG2KWHyxE3@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-102283-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102283 --- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> --- (In reply to Giuseppe D'Angelo from comment #2) > Hi, > > Do you think that in my original testcase the call should be rejected as > ambiguous as well? (It seems "reasonable" to me, but maybe I'm missing some > niche detail about overload resolution when combined with template > deduction.) > > > This small variation over the testcase: > > > struct A { }; > struct B { }; > > template <typename T = A> > void f(T &&); // #1 > void f(const B&) = delete; // #2 > > int main() { > f({}); > } > > This now makes GCC select #2, and fail to compile because it's deleted; I think this is because GCC considers the two ICSes to be incomparable, but then #2 wins over #1 anyway because non-templates are preferred over templates (as per [over.match.best]). Whereas with Clang/MSVC, I presume they consider the ICS for #1 to be better than that for #2 (though I haven't found the wording in the standard that supports this), so #1 wins during the ICS comparison stage of overload resolution. > Clang and MSVC still select #1. But a further, minor change: > > > struct A { }; > struct B { }; > > template <typename T = int> // <-- changed this > void f(T &&); // #1 > void f(const B&) = delete; // #2 > > int main() { > f({}); > } > > makes GCC select #1... This example is a different because the ICS for #1 is a standard conversion sequence (since the {} initializes a non-class) whereas for #2 it's a user-defined conversion sequence (since the {} initializes a class), and the former kind is always better than the latter. So #1 wins due to having the better ICS.
next prev parent reply other threads:[~2021-09-16 13:40 UTC|newest] Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-09-10 19:06 [Bug c++/102283] New: " dangelog at gmail dot com 2021-09-15 16:16 ` [Bug c++/102283] " ppalka at gcc dot gnu.org 2021-09-15 20:29 ` dangelog at gmail dot com 2021-09-16 13:40 ` ppalka at gcc dot gnu.org [this message] 2021-09-16 17:46 ` jason at gcc dot gnu.org 2021-10-01 17:30 ` dangelog at gmail dot com 2021-10-12 13:40 ` ppalka at gcc dot gnu.org 2021-10-12 13:47 ` ppalka at gcc dot gnu.org
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