Compiler problem with MSVC and std:c++20

[dirk-zimoch]

When compiling with MSVC in c++20 mode (options -Zc:__cplusplus -std:c++20) building pvUnitTest.cpp fails:

pvData\src\misc\pv\sharedVector.h(294): error C2182: 'reference': illegal use of type 'void'
pvData\src\misc\pv\sharedVector.h(706): note: see reference to class template instantiation 'epics::pvData::shared_vector<void,void>' being compiled
pvData\src\misc\pv\sharedVector.h(749): note: see reference to class template instantiation 'epics::pvData::shared_vector<E,meta::is_void<E,void>::type>' being compiled
pvData\src\misc\pv\sharedVector.h(295): error C2182: 'const_reference': illegal use of type 'void'
pvData\src\misc\pv\sharedVector.h(334): error C2182: 'e': illegal use of type 'void'
pvData\src\misc\pv\sharedVector.h(498): error C2182: 'v': illegal use of type 'void'
pvData\src\misc\pv\sharedVector.h(602): error C2182: 'v': illegal use of type 'void'
pvData\src\misc\pv\sharedVector.h(708): error C2039: 'm_vtype': is not a member of 'epics::pvData::shared_vector<void,void>'
pvData\src\misc\pv\sharedVector.h(706): note: see declaration of 'epics::pvData::shared_vector<void,void>'
pvData\src\misc\pv\sharedVector.h(713): error C2039: 'm_vtype': is not a member of 'epics::pvData::shared_vector<const void,void>'
pvData\src\misc\pv\sharedVector.h(711): note: see declaration of 'epics::pvData::shared_vector<const void,void>'
pvData\src\pv\pvData.h(539): warning C4250: 'epics::pvData::PVString': inherits 'epics::pvData::PVScalarValue<std::string>::epics::pvData::PVScalarValue<std::string>::deserialize' via dominance
pvData\src\pv\pvData.h(440): note: see declaration of 'epics::pvData::PVScalarValue<std::string>::deserialize'

[dirk-zimoch]

See also https://github.com/dirk-zimoch/epics-base/actions/runs/18224396535/job/51891777002

[mdavidsaver]

It looks like the void specialization of sharedVector is not being (un)selected correctly. I suspect the root issue is with my meta::is_void<E>.

pvDataCPP/src/misc/pv/sharedVector.h

Lines 653 to 655 in b3a1077

	template<typename E>
	class shared_vector<E, typename meta::is_void<E>::type >
	: public detail::shared_vector_base<E>

pvDataCPP/src/misc/pv/templateMeta.h

Lines 104 to 106 in b3a1077

	template<typename T, class R = void> struct is_void {};
	template<class R> struct is_void<void,R> { typedef R type; };
	template<class R> struct is_void<const void,R> { typedef R type; };

There might be a simple fix to conditionally use std::is_void as I do not think I used the second R parameter anywhere.

#if __cplusplus>=201103L
using std::is_void;
#else
 template<typename T, class R = void> struct is_void {}; 
 template<class R> struct is_void<void,R> { typedef R type; }; 
 template<class R> struct is_void<const void,R> { typedef R type; }; 
#endif

[mdavidsaver]

nope... A vastly simplified case compiles just fine. So the issue is not with is_void, but somewhere in the body.

namespace meta {
template<typename T, class R = void> struct is_void {};
template<class R> struct is_void<void,R> { typedef R type; };
template<class R> struct is_void<const void,R> { typedef R type; };
}

namespace detail {
    template<typename E>
    class shared_vector_base
    {};
}

template<typename E, class Enable>
class shared_vector : public detail::shared_vector_base<E>
{};

template<typename E>
class shared_vector<E, typename meta::is_void<E>::type >
    : public detail::shared_vector_base<E>
    {};

[mdavidsaver]

So I think the important error is coming from line 708. The member m_vtype is only present for the void specialization. For reasons I do not immediately understand shared_vector<void>& does not seem to select the void specialization.

pvData\src\misc\pv\sharedVector.h(708): error C2039: 'm_vtype': is not a member of 'epics::pvData::shared_vector<void,void>'
pvData\src\misc\pv\sharedVector.h(706): note: see declaration of 'epics::pvData::shared_vector<void,void>'

    shared_vector(shared_vector<void>& O,
                  detail::_shared_vector_freeze_tag t)
        :base_t(O,t), m_vtype(O.m_vtype)  // <-- line 708
    {}

pvDataCPP/src/misc/pv/sharedVector.h

Lines 706 to 709 in b3a1077

	shared_vector(shared_vector<void>& O,
	detail::_shared_vector_freeze_tag t)
	:base_t(O,t), m_vtype(O.m_vtype)
	{}

fyi. Errors with -Zc:__cplusplus -std:c++20 but not -Zc:__cplusplus -std:c++17 for the same vs2019.

[mdavidsaver]

This does seem to be something specific to the interpretation of c++20 (and maybe later) by MSVC. The combinations of clang/gcc and language version I tried with godbolt.org all compile. As does MSVC with c++17.

In the short term I can only recommend limiting MSVC builds to c++17.

[dirk-zimoch]

The combinations of clang/gcc and language version I tried with godbolt.org all compile. As does MSVC with c++17.

Yes, gcc and clang accept the code with c++20 and c++2a. Only MSVC with c++20 (+?) complains.