[cfe-dev] Fixes to partial ordering break Boost.Hana

[Richard Smith via cfe-dev]

I reverted the problematic cases of r290511 in r292183.

On 16 January 2017 at 12:09, Mehdi Amini <mehdi.amini at apple.com> wrote:

> CC Richard who committed this in the first place, and Hans to track this
> as a potential release blocker.
>
> —
> Mehdi
>
>
> > On Jan 16, 2017, at 11:17 AM, Pavel A. Lebedev via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
> >
> > Hi,
> >
> > Any comments on this?
> > As it stands, clang 4 will be unusable with Boost.Hana,
> > at least the already released versions.
> > In case this is actually the library's fault, this is being discussed at
> > https://github.com/boostorg/hana/issues/318
> >
> >> -----Original Message-----
> >> From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of
> Pavel A. Lebedev via cfe-dev
> >> Sent: Tuesday, January 10, 2017 2:21 AM
> >> To: 'Clang Dev' <cfe-dev at lists.llvm.org>
> >> Subject: [cfe-dev] Fixes to partial ordering break Boost.Hana
> >>
> >> Hi.
> >>
> >> Commit r290511 "Fix some subtle wrong partial ordering bugs particularly
> >> with C++1z auto-typed non-type template parameters"
> >> seems to break some template specialization tricks at the core of
> >> Boost.Hana
> >> library.
> >> The reduced example is:
> >>
> >> $ cat hana.cpp
> >>
> >> // SFINAE helpers.
> >>
> >> template<bool Condition>
> >> struct when;
> >>
> >> template<typename...>
> >> struct is_valid
> >> {
> >>    static constexpr bool value = true;
> >> };
> >>
> >> // A class and its tag.
> >>
> >> struct c_tag;
> >>
> >> struct C
> >> {
> >>    using tag = c_tag;
> >> };
> >>
> >> // Tag getter. When a type has a nested type "tag", it specifies its
> tag.
> >> // When it doesn't have one, the tag is type itself.
> >>
> >> template<typename T,typename = void>
> >> struct tag_of : tag_of<T,when<true>> {};
> >>
> >> template<typename T,bool Condition>
> >> struct tag_of<T,when<Condition>>
> >> {
> >>    using type = T;
> >> };
> >>
> >> template<typename T>
> >> struct tag_of<T,when<is_valid<typename T::tag>::value>>
> >> {
> >>    using type = typename T::tag;
> >> };
> >>
> >> // Attempt to calculate tag of C.
> >>
> >> using tag_of_c = typename tag_of<C>::type;
> >>
> >>
> >> $ clang++ -std=c++14 -fsyntax-only hana.cpp
> >>
> >> hana.cpp:25:17: error: ambiguous partial specializations of 'tag_of<C,
> >> when<true> >'
> >> struct tag_of : tag_of<T,when<true>> {};
> >>                ^
> >> hana.cpp:41:27: note: in instantiation of template class 'tag_of<C,
> >> void>'
> >> requested here
> >> using tag_of_c = typename tag_of<C>::type;
> >>                          ^
> >> hana.cpp:28:8: note: partial specialization matches [with T = C,
> >> Condition =
> >> true]
> >> struct tag_of<T,when<Condition>>
> >>       ^
> >> hana.cpp:34:8: note: partial specialization matches [with T = C]
> >> struct tag_of<T,when<is_valid<typename T::tag>::value>>
> >>       ^
> >> 1 error generated.
> >>
> >>
> >> Is this a problem with the library or clang?
> >
> > ---
> > Pavel A. Lebedev
> >
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
>
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