Compilation error with MSVC 2022

[mglisse]

#include <boost/unordered/unordered_flat_set.hpp>
struct A
{
    struct H { size_t operator()(const int* c) const; };
    using C = boost::unordered_flat_set<int*, H>;
    using I = C::iterator;
};
int main(){}

Compiling this with Visual Studio 2022 (17.8.5) and Boost 1.85 yields

1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\table.hpp(348,32): error C2247: 'A::H' not accessible because 'boost::empty_::empty_value<Hash,0,true>' uses 'private' to inherit from 'A::H'
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\table.hpp(348,32): error C2247:         with
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\table.hpp(348,32): error C2247:         [
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\table.hpp(348,32): error C2247:             Hash=A::H
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\table.hpp(348,32): error C2247:         ]
1>(compiling source file 'ConsoleApplication1.cpp')
1>C:\Users\ci\source\repos\ConsoleApplication1\ConsoleApplication1\ConsoleApplication1.cpp(4,12):
1>see declaration of 'A::H'
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\core.hpp(1361,12):
1>see declaration of 'boost::empty_::empty_value<Hash,0,true>'
1>        with
1>        [
1>            Hash=A::H
1>        ]
1>C:\Users\ci\source\repos\ConsoleApplication1\ConsoleApplication1\ConsoleApplication1.cpp(4,12):
1>see declaration of 'A::H'
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\table.hpp(348,32):
1>the template instantiation context (the oldest one first) is
1>	C:\Users\ci\source\repos\ConsoleApplication1\ConsoleApplication1\ConsoleApplication1.cpp(6,26):
1>	see reference to class template instantiation 'boost::unordered::unordered_flat_set<int *,A::H,std::equal_to<Key>,std::allocator<Key>>' being compiled
1>        with
1>        [
1>            Key=int *
1>        ]
1>	C:\vcpkg\installed\x64-windows\include\boost\unordered\unordered_flat_set.hpp(48,18):
1>	see reference to class template instantiation 'boost::unordered::detail::foa::table<boost::unordered::detail::foa::flat_set_types<Key>,Hash,KeyEqual,std::allocator<Key>>' being compiled
1>        with
1>        [
1>            Key=int *,
1>            Hash=A::H,
1>            KeyEqual=std::equal_to<int *>
1>        ]
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\table.hpp(348,32): error C2248: 'A::H': cannot access private struct declared in class 'boost::empty_::empty_value<Hash,0,true>'
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\table.hpp(348,32): error C2248:         with
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\table.hpp(348,32): error C2248:         [
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\table.hpp(348,32): error C2248:             Hash=A::H
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\table.hpp(348,32): error C2248:         ]
1>(compiling source file 'ConsoleApplication1.cpp')
1>C:\Users\ci\source\repos\ConsoleApplication1\ConsoleApplication1\ConsoleApplication1.cpp(4,12):
1>see declaration of 'A::H'
1>C:\vcpkg\installed\x64-windows\include\boost\unordered\detail\foa\core.hpp(1361,12):
1>see declaration of 'boost::empty_::empty_value<Hash,0,true>'
1>        with
1>        [
1>            Hash=A::H
1>        ]

This is most likely a bug in visual studio (it might even be fixed already in more recent versions, I don't have other versions at hand right now), but in case you want to work around it in Boost somehow...

[k3DW]

It looks like this isn't specifically an issue in Unordered. I reproduced this using only a stand-in for boost::empty_value. It's still happening on the latest Visual Studio 2022, 17.10.3. The Unordered code does basically the following.

// A stand-in for boost::empty_value
template <class T>
class empty_value : T
{
public:
    using type = T;
};

template <class T>
struct S : empty_value<T>
{
    using type = typename empty_value<T>::type;
};

struct Outer { struct Inner {}; };

S<Outer::Inner> s{};

int main(){}

This clearly looks like a compiler bug in MSVC. It could be fixed by inheriting publicly from T in empty_value. I'm not sure if this is a breaking change in Core. There is no workaround I could see in the Unordered library. I'll look into changing this in Core.

For your use case, you can work around this problem by moving H outside of A. It seems like MSVC has a problem with nested classes/structs in this case.

[joaquintides]

Hi @mglisse can you confirm if this fix solves your original problem? Thank you!

[mglisse]

Thanks. I'll try to check if it fixes the original problem at some point, but don't hold your breath (I don't have the environment available anymore). If it fixes the reduced testcase, I think that's good enough to close the issue, and I'll open a new one later if it turns out not to be sufficient for our full code.