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

[Richard Smith via cfe-dev]

This looks like a bug in Clang to me. I'll investigate tomorrow.

On 16 Jan 2017 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
>
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