On Mon, 7 Nov 2022 at 22:49, Jason Merrill <ja...@redhat.com> wrote: > > On 11/7/22 12:04, Jonathan Wakely wrote: > > On Mon, 7 Nov 2022 at 21:56, Jonathan Wakely <jwak...@redhat.com> wrote: > >> > >> On Mon, 7 Nov 2022 at 20:58, Jason Merrill <ja...@redhat.com> wrote: > >>> > >>> Tested x86_64-pc-linux-gnu. Jonathan, what do you want to do about the > >>> library > >>> test failure? > >>> > >>> -- >8 -- > >>> > >>> This paper is resolving the problem of well-formed C++17 code becoming > >>> ambiguous in C++20 due to asymmetrical operator== being compared with > >>> itself > >>> in reverse. I had previously implemented a tiebreaker such that if the > >>> two > >>> candidates were functions with the same parameter types, we would prefer > >>> the > >>> non-reversed candidate. But the committee went with a different approach: > >>> if there's an operator!= with the same parameter types as the operator==, > >>> don't consider the reversed form of the ==. > >>> > >>> So this patch implements that, and changes my old tiebreaker to give a > >>> pedwarn if it is used. I also noticed that we were giving duplicate > >>> errors > >>> for some testcases, and fixed the tourney logic to avoid that. > >>> > >>> As a result, a lot of tests of the form > >>> > >>> struct A { bool operator==(const A&); }; > >>> > >>> need to be fixed to add a const function-cv-qualifier, e.g. > >>> > >>> struct A { bool operator==(const A&) const; }; > >>> > >>> The committee thought such code ought to be fixed, so breaking it was > >>> fine. > >>> > >>> 18_support/comparisons/algorithms/fallback.cc also breaks with this patch, > >>> because of the similarly asymmetrical > >>> > >>> bool operator==(const S&, S&) { return true; } > >>> > >>> I assume this was written this way deliberately, so I'm not sure what to > >>> do > >>> about it. > >> > >> Yes, that was deliberate. The compare_strong_order_fallback function > >> has these constraints: > >> > >> template<typename _Tp, __decayed_same_as<_Tp> _Up> > >> requires __strongly_ordered<_Tp, _Up> || __op_eq_lt<_Tp, _Up> > >> constexpr strong_ordering > >> operator() [[nodiscard]] (_Tp&& __e, _Up&& __f) const > >> > >> And similarly for the other fallbacks. So I wanted to check that two > >> types that decay to the same type (like S and const S) can be compared > >> with == and <, and therefore can be used with this function. > >> > >> But if such asymmetry is no longer allowed, maybe the library function > >> is no longer usable with pathological cases like the test, and so the > >> test should be changed. We can't just replace the decayed_same_as > >> constraint with same_as because the std::strong_order customization > >> point still supports similar types, but we could do: > >> > >> --- a/libstdc++-v3/libsupc++/compare > >> +++ b/libstdc++-v3/libsupc++/compare > >> @@ -1057,11 +1057,11 @@ namespace std _GLIBCXX_VISIBILITY(default) > >> }; > >> > >> template<typename _Tp, typename _Up> > >> - concept __op_eq_lt = requires(_Tp&& __t, _Up&& __u) > >> + concept __op_eq_lt = same_as<_Tp, _Up> && requires(_Tp&& __t) > >> { > >> - { static_cast<_Tp&&>(__t) == static_cast<_Up&&>(__u) } > >> + { static_cast<_Tp&&>(__t) == static_cast<_Tp&&>(__t) } > >> -> convertible_to<bool>; > >> - { static_cast<_Tp&&>(__t) < static_cast<_Up&&>(__u) } > >> + { static_cast<_Tp&&>(__t) < static_cast<_Tp&&>(__t) } > >> -> convertible_to<bool>; > >> }; > > > > No wait, that's nonsense. We can still try to compare similar types, > > it's just that they won't be comparable unless their comparison ops > > have two parameters of the same type. > > Basically, though in this case the problem is that the arguments are the > same type and the parameters are different.
Ah, so the operator== isn't actually rejected (despite being pathologically dumb) it's just that some uses of it result in ambiguities. > >> And then adjust the test accordingly. If those fallbacks can no longer > >> support mixed types, does the resolution of > >> https://cplusplus.github.io/LWG/issue3465 even make sense now? If E > >> and F must be the same type now, then E < F already implies F < E. I > >> think we need some library changes to sync with P2468R2. > > > > I think this bit was right though. E and F might be different types, > > but E < F implies F < E. Is that right? > > The P2468 changes only affect ==/!=, not <, so LWG3465 should be unaffected. Gotcha. > I think the only problem is the test itself: the <S,S> asserts need to > be inverted because S and S cannot be compared for equality with the > asymmetrical op== due to the normal candidate being better for arg 2 and > the reversed candidate being better for arg 1. The <const S, S> asserts > are fine because the arguments match the asymmetry, so the normal > candidate is better for both args. Gotcha. > So, the below fixes the test, does > it make sense to you? Yep, looks good. Thanks.