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

[Hans Wennborg via cfe-dev]

Merged in r292338.

Cheers,
Hans

On Tue, Jan 17, 2017 at 11:08 AM, Richard Smith <richard at metafoo.co.uk> wrote:
> I reverted the problematic cases of r290511 in r292183.
>
> Hans, that change should go onto the Clang 4 branch.
>
>
> 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
>>
>