public inbox for gcc-bugs@sourceware.org help / color / mirror / Atom feed
From: "waffl3x at protonmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/113191] New: [10.1/11/12/13/14 Regression] Incorrect overload resolution when base class function introduced with a using declaration is more constrained than a function declared in the derived class Date: Mon, 01 Jan 2024 17:00:45 +0000 [thread overview] Message-ID: <bug-113191-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113191 Bug ID: 113191 Summary: [10.1/11/12/13/14 Regression] Incorrect overload resolution when base class function introduced with a using declaration is more constrained than a function declared in the derived class Product: gcc Version: 10.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: waffl3x at protonmail dot com Target Milestone: --- https://godbolt.org/z/91esEGhj4 template<typename = void> struct B { constexpr int f() requires true { return 5; } }; template<typename = void> struct S : B<> { using B::f; constexpr int f() { return 10; } }; static_assert(S<>{}.f() == 5); The bug does not occur in this case: https://godbolt.org/z/dPM1Gfc1c We do the right thing in more_specialized_fn (which is why the second case works fine), but that doesn't apply in this case. Perhaps we should be lifting that work from more_specialized_fn to joust? Unfortunately, the changes in more_specialized_fn do not properly handle the following case. struct B { template<typename T> requires true int g(T) { return 5; } }; struct S : B { using B::g; template<typename T> int g(this S&, T) { return 10; } }; int main() { S s{}; s.g(0); } This case is ambiguous, I believe the main issue is that more_specialized_fn does not implement [over.match.funcs.general.4]. This is kind of a separate bug but they are connected, and it's relevant to how we decide to fix it. I'm mildly of the opinion that we should be rewriting iobj member functions that are introduced with a using declaration to have an object parameter matching that of the class it was introduced into. This might open a can of worms, but it more closely matches the behavior specified by the standard. [over.match.funcs.general.4] For non-conversion functions that are implicit object member functions nominated by a using-declaration in a derived class, the function is considered to be a member of the derived class for the purpose of defining the type of the implicit object parameter. This wasn't really as relevant before, but it does become relevant now because of the following case. https://godbolt.org/z/MjP5nrd8q template<typename = void> struct S; template<typename = void> struct B { constexpr int f(this S<> const&) { return 5; } constexpr int g() const { return 5; } }; template<typename> struct S : B<> { using B<>::f; using B<>::g; constexpr int f() const { return 10; } constexpr int g(this S const&) { return 10; } }; inline constexpr S<> s{}; static_assert(s.f() == 5); static_assert(s.g() == 5); I am not 100% sure what the correct behavior here is, but my interpretation is that the constraints should be taken into account. Again, this is slightly unrelated to this bug report, but it's more evidence that we should just overhaul everything with iobj member functions, and follow the standard to the letter. I think it's going to be simpler in the long run, trying to hack it in this way or that is just going to keep introducing problems. With that said, I recognize theres potential implementation difficulties with doing it this way too. Ultimately, it's a big decision so I don't mean to declare that we need to do it this way, I merely intend to present it as food for thought. My implementation currently does not do either of these correct at all, and as you can see in the godbolt link, clang does not exhibit the behavior I believe to be correct either. One last note, despite this being a regression, I don't believe that the previous implementation will be ideal (not that I've found the divergence yet.) Previous versions had the liberty of making different assumptions, and as demonstrated in the examples with xobj member functions, we have some new issues we need to work around here as well. I've spent the better part of 6 hours investigating this issue and the issues related to it, trying to figure out how to handle it for my patch. I have concluded that I'm not going to try to fix this bug for xobj member functions, and instead going to wait for this bug to be fixed to try to handle it. So the behavior for xobj member functions and iobj member functions will both be equally incorrect. Anyway, since I have spent so much time staring at this I might have made some mistakes in this report, or it will just be more confusing and disjointed than I hoped. Hopefully not though!
next reply other threads:[~2024-01-01 17:00 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2024-01-01 17:00 waffl3x at protonmail dot com [this message] 2024-01-02 15:49 ` [Bug c++/113191] [11/12/13/14 " ppalka at gcc dot gnu.org 2024-01-02 18:24 ` waffl3x at protonmail dot com 2024-01-11 22:02 ` cvs-commit at gcc dot gnu.org 2024-01-12 15:14 ` jason at gcc dot gnu.org
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=bug-113191-4@http.gcc.gnu.org/bugzilla/ \ --to=gcc-bugzilla@gcc.gnu.org \ --cc=gcc-bugs@gcc.gnu.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).