Re: [boost] [poly_collection][VS 2017] Potential compiler bug

Subject: Re: [boost] [poly_collection][VS 2017] Potential compiler bug
From: Stephan T. Lavavej (stl_at_[hidden])
Date: 2017-06-02 20:54:08

Yep, looks like a compiler bug. Reduced (see below) and filed as VSO#445619.

This works if you call std::all_of() without explicit template arguments - I'm not sure why you were trying to provide them.

Reduced repro:

C:\Temp>type meow.cpp
namespace Std {
    template <typename InIt, typename Pr> bool AllOf(InIt, InIt, Pr) {
        return true;
    }

    template <typename T> struct RemoveRef {
        using type = T;
    };
    template <typename T> struct RemoveRef<T&> {
        using type = T;
    };
    template <typename T> struct RemoveRef<T&&> {
        using type = T;
    };

    template <typename T> constexpr T&& Forward(typename RemoveRef<T>::type& t) noexcept {
        return static_cast<T&&>(t);
    }
}

template <typename... Ts> struct Meow {
    template <typename... Args> auto operator()(Args&&... args) const
    -> decltype(Std::AllOf<Ts...>(Std::Forward<Args>(args)...)) {
         return Std::AllOf<Ts...>(Std::Forward<Args>(args)...);
    }
};

int main() {
    Meow<> alg;
    alg(10, 10, 3.14);
}

C:\Temp>cl /EHsc /nologo /W4 meow.cpp
meow.cpp
meow.cpp(23): error C3546: '...': there are no parameter packs available to expand
meow.cpp(29): note: see reference to class template instantiation 'Meow<>' being compiled

C:\Temp>clang-cl /EHsc /nologo /W4 meow.cpp

C:\Temp>

Thanks,
STL

-----Original Message-----
From: Boost [mailto:boost-bounces_at_[hidden]] On Behalf Of Joaquin M López Muñoz via Boost
Sent: Friday, June 2, 2017 9:02 AM
To: boost_at_[hidden]
Cc: Joaquin M López Muñoz <joaquinlopezmunoz_at_[hidden]>
Subject: [boost] [poly_collection][VS 2017] Potential compiler bug

Hi,

Boost.PolyCollection tests have started to cycle and I found VS 2017 aka MSVC 14.1 fails at test_algorithm.cpp:

   https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Ftinyurl.com%2Fyb4zdkys&data=02%7C01%7Cstl%40exchange.microsoft.com%7Cf1f8085f63704a3d13f408d4a9d0d4b0%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636320161769104283&sdata=lmdoZRJng1ZpCUv9w16gOsR7UAD%2BCuIcmYeUDN8ah50%3D&reserved=0

whereas this test compiles and runs OK with VS 2015 aka MSVC 4.0 (_MSC_FULL_VER = 190023026). I've distilled down the problem to this test case:

     #include <algorithm>
     #include <utility>

     template<typename... Ts>
     struct all_of
     {
       template<typename... Args>
       auto operator()(Args&&... args)const->
decltype(std::all_of<Ts...>(std::forward<Args>(args)...))
       {
         return std::all_of<Ts...>(std::forward<Args>(args)...);
       }
     };

     int main()
     {
       int x;
       all_of<> alg;
       alg(&x,&x,[](int){return true;});
     }

which works fine with GCC and VS 2015 but fails in webcompiler.cloudapp.net with

   Compiled with  /EHsc /nologo /W4 /c
   main.cpp
   main.cpp(9): error C3546: '...': there are no parameter packs available to expand
   main.cpp(18): note: see reference to class template instantiation 'all_of<>' being compiled

(_MSC_FULL_VER = 191125331, which I think is VS 2017.3 or something).

This looks like a regression to me. Deleting the ->decltype(...) bit makes the problem go away but I need to stay within C+11.

* Can someone (STL?) confirm this is indeed a compiler bug?
* Any rewording of the code that might be generally applicable to C++11 so that I don't have to include a specific workaround for VS 2017?

Thank you,

Joaquín M López Muñoz


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