flat_for_each_field not working on MSVC

[janwilmans]

#include <boost/pfr/flat/core.hpp>

int main() {
    struct nested { int i; short data[3]; };
    struct foo { int i; nested n; };
    std::size_t sum = 0;
    boost::pfr::flat_for_each_field(foo{1, {2, {3, 4, 5}}}, [&sum](auto v) {
        sum += v;
    });
}

This example does not compile, even with /std:c++17 and without /permissive-.

Reproduce: just paste the code in a new console project, set it to c++17 and conformance mode to 'no'

This is the error message:

1>    C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\bin\HostX86\x64\CL.exe /c /Imagic_get\include /Zi /nologo /W3 /WX- /diagnostics:classic /sdl /O2 /Oi /GL /D NDEBUG /D _CONSOLE /D _UNICODE /D UNICODE /Gm- /EHsc /MD /GS /Gy /fp:precise /Zc:wchar_t /Zc:forScope /Zc:inline /std:c++17 /Yu"pch.h" /Fp"x64\Release\ConsoleApplication10.pch" /Fo"x64\Release\\" /Fd"x64\Release\vc141.pdb" /Gd /TP /FC /errorReport:prompt ConsoleApplication10.cpp
1>    Tracking command:
1>    C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\15.0\Bin\Tracker.exe /d "C:\Program Files (x86)\MSBuild\15.0\FileTracker\FileTracker32.dll" /i "C:\Users\Jan Wilmans\source\repos\ConsoleApplication10\ConsoleApplication10\x64\Release\ConsoleA.D2A74CDF.tlog" /r "C:\USERS\JAN WILMANS\SOURCE\REPOS\CONSOLEAPPLICATION10\CONSOLEAPPLICATION10\CONSOLEAPPLICATION10.CPP" /b MSBuildConsole_CancelEvent3fe69d296dda41e989270a8edadfde3e  /c "C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\VC\Tools\MSVC\14.16.27023\bin\HostX86\x64\CL.exe"  /c /Imagic_get\include /Zi /nologo /W3 /WX- /diagnostics:classic /sdl /O2 /Oi /GL /D NDEBUG /D _CONSOLE /D _UNICODE /D UNICODE /Gm- /EHsc /MD /GS /Gy /fp:precise /Zc:wchar_t /Zc:forScope /Zc:inline /std:c++17 /Yu"pch.h" /Fp"x64\Release\ConsoleApplication10.pch" /Fo"x64\Release\\" /Fd"x64\Release\vc141.pdb" /Gd /TP /FC /errorReport:prompt ConsoleApplication10.cpp
1>    ConsoleApplication10.cpp
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(103): error C3779: 'boost::pfr::detail::loophole': a function that returns 'auto' cannot be used before it is defined
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(59): note: see declaration of 'boost::pfr::detail::loophole'
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(162): note: see reference to class template instantiation 'boost::pfr::detail::loophole_type_list_lref<T,U>' being compiled
1>            with
1>            [
1>                T=type,
1>                U=indexes
1>            ]
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(237): note: see reference to function template instantiation 'auto boost::pfr::detail::tie_as_tuple_loophole_impl<T>(T &) noexcept' being compiled
1>            with
1>            [
1>                T=main::foo
1>            ]
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\core.hpp(133): note: see reference to function template instantiation 'auto boost::pfr::detail::tie_as_flat_tuple<T>(T &)' being compiled
1>            with
1>            [
1>                T=main::foo
1>            ]
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\consoleapplication10.cpp(11): note: see reference to function template instantiation 'void boost::pfr::flat_for_each_field<main::foo,main::<lambda_611397c7fcdc2babe31551a0ec0cc907>>(T &&,F &&)' being compiled
1>            with
1>            [
1>                T=main::foo,
1>                F=main::<lambda_611397c7fcdc2babe31551a0ec0cc907>
1>            ]
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(128): error C3779: 'boost::pfr::detail::loophole': a function that returns 'auto' cannot be used before it is defined
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(59): note: see declaration of 'boost::pfr::detail::loophole'
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(166): error C3203: 'tuple': unspecialized class template can't be used as a template argument for template parameter 'S', expected a real type
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\offset_based_getter.hpp(63): error C2338: ====================> Boost.PFR: Member sequence does not indicate correct size for struct type!
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(166): note: see reference to class template instantiation 'boost::pfr::detail::offset_based_getter<type,int>' being compiled
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(168): error C2955: 'boost::pfr::detail::sequence_tuple::tuple': use of class template requires template argument list
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\sequence_tuple.hpp(78): note: see declaration of 'boost::pfr::detail::sequence_tuple::tuple'
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(168): error C2512: 'std::integral_constant<size_t,Index>': no appropriate default constructor available
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(168): note: The target type has no constructors
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(164): error C2672: 'boost::pfr::detail::make_flat_tuple_of_references': no matching overloaded function found
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(169): error C2780: 'auto boost::pfr::detail::make_flat_tuple_of_references(TupleOrUserType &,const Getter &,std::integral_constant<size_t,Index>,std::integral_constant<size_t,1>) noexcept': expects 4 arguments - 3 provided
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\make_flat_tuple_of_references.hpp(40): note: see declaration of 'boost::pfr::detail::make_flat_tuple_of_references'
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(169): error C2780: 'boost::pfr::detail::sequence_tuple::tuple<> boost::pfr::detail::make_flat_tuple_of_references(TupleOrUserType &,const Getter &,std::integral_constant<size_t,Index>,std::integral_constant<size_t,0>) noexcept': expects 4 arguments - 3 provided
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\make_flat_tuple_of_references.hpp(37): note: see declaration of 'boost::pfr::detail::make_flat_tuple_of_references'
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(169): error C2780: 'auto boost::pfr::detail::make_flat_tuple_of_references(TupleOrUserType &,const Getter &,std::integral_constant<size_t,Index>,std::integral_constant<size_t,Index>) noexcept': expects 4 arguments - 3 provided
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\make_flat_tuple_of_references.hpp(34): note: see declaration of 'boost::pfr::detail::make_flat_tuple_of_references'
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(236): error C2672: 'boost::pfr::detail::tie_as_tuple_recursively': no matching overloaded function found
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(238): error C2893: Failed to specialize function template 'auto boost::pfr::detail::tie_as_tuple_recursively(T &&) noexcept'
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(238): note: With the following template arguments:
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(238): note: 'T=void'
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(241): error C3536: 'rec_tuples': cannot be used before it is initialized
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(241): error C2651: 'int': left of '::' must be a class, struct or union
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(241): error C2062: type 'unknown-type' unexpected
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(241): error C2039: 'size_v': is not a member of '`global namespace''
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(241): error C2143: syntax error: missing ';' before '{'
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(242): error C2059: syntax error: ')'
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\core.hpp(133): error C3313: 'tup': variable cannot have the type 'void'
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\core.hpp(135): error C3536: 'tup': cannot be used before it is initialized
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(26): error C2651: 'void': left of '::' must be a class, struct or union
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(36): note: see reference to alias template instantiation 'boost::pfr::flat_tuple_size<main::foo>' being compiled
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\core.hpp(137): note: see reference to variable template 'const size_t flat_tuple_size_v<`main'::`2'::foo>' being compiled
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(26): error C2062: type 'unknown-type' unexpected
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(26): error C2039: 'size_v': is not a member of '`global namespace''
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(36): error C2938: 'boost::pfr::flat_tuple_size<main::foo>' : Failed to specialize alias template
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(36): error C2062: type 'unknown-type' unexpected
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(36): error C2039: 'value': is not a member of '`global namespace''
1>    c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\core.hpp(137): error C3376: 'boost::pfr::flat_tuple_size_v': only static data member templates are allowed
1>    The command exited with code 2.
1>  Done executing task "CL" -- FAILED.
1>Done building target "ClCompile" in project "ConsoleApplication10.vcxproj" -- FAILED.
1>
1>Done building project "ConsoleApplication10.vcxproj" -- FAILED.
1>
1>Build FAILED.
1>
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(103): error C3779: 'boost::pfr::detail::loophole': a function that returns 'auto' cannot be used before it is defined
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(128): error C3779: 'boost::pfr::detail::loophole': a function that returns 'auto' cannot be used before it is defined
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(166): error C3203: 'tuple': unspecialized class template can't be used as a template argument for template parameter 'S', expected a real type
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\offset_based_getter.hpp(63): error C2338: ====================> Boost.PFR: Member sequence does not indicate correct size for struct type!
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(168): error C2955: 'boost::pfr::detail::sequence_tuple::tuple': use of class template requires template argument list
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(168): error C2512: 'std::integral_constant<size_t,Index>': no appropriate default constructor available
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(164): error C2672: 'boost::pfr::detail::make_flat_tuple_of_references': no matching overloaded function found
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(169): error C2780: 'auto boost::pfr::detail::make_flat_tuple_of_references(TupleOrUserType &,const Getter &,std::integral_constant<size_t,Index>,std::integral_constant<size_t,1>) noexcept': expects 4 arguments - 3 provided
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(169): error C2780: 'boost::pfr::detail::sequence_tuple::tuple<> boost::pfr::detail::make_flat_tuple_of_references(TupleOrUserType &,const Getter &,std::integral_constant<size_t,Index>,std::integral_constant<size_t,0>) noexcept': expects 4 arguments - 3 provided
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(169): error C2780: 'auto boost::pfr::detail::make_flat_tuple_of_references(TupleOrUserType &,const Getter &,std::integral_constant<size_t,Index>,std::integral_constant<size_t,Index>) noexcept': expects 4 arguments - 3 provided
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(236): error C2672: 'boost::pfr::detail::tie_as_tuple_recursively': no matching overloaded function found
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(238): error C2893: Failed to specialize function template 'auto boost::pfr::detail::tie_as_tuple_recursively(T &&) noexcept'
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(241): error C3536: 'rec_tuples': cannot be used before it is initialized
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(241): error C2651: 'int': left of '::' must be a class, struct or union
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(241): error C2062: type 'unknown-type' unexpected
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(241): error C2039: 'size_v': is not a member of '`global namespace''
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(241): error C2143: syntax error: missing ';' before '{'
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\detail\core14_loophole.hpp(242): error C2059: syntax error: ')'
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\core.hpp(133): error C3313: 'tup': variable cannot have the type 'void'
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\core.hpp(135): error C3536: 'tup': cannot be used before it is initialized
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(26): error C2651: 'void': left of '::' must be a class, struct or union
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(26): error C2062: type 'unknown-type' unexpected
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(26): error C2039: 'size_v': is not a member of '`global namespace''
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(36): error C2938: 'boost::pfr::flat_tuple_size<main::foo>' : Failed to specialize alias template
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(36): error C2062: type 'unknown-type' unexpected
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\tuple_size.hpp(36): error C2039: 'value': is not a member of '`global namespace''
1>c:\users\jan wilmans\source\repos\consoleapplication10\consoleapplication10\magic_get\include\boost\pfr\flat\core.hpp(137): error C3376: 'boost::pfr::flat_tuple_size_v': only static data member templates are allowed
1>    0 Warning(s)
1>    27 Error(s)
1>
1>Time Elapsed 00:00:00.52
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

[MKlimenko]

@janwilmans, according to the documentation:

MSVC currently supports only precise functions and only in /std:c++latest or /std:c++17 modes.

[janwilmans]

I'm not sure what that means? what are precise functions and get we work around them?

[MKlimenko]

There's a code example:

#include <boost/pfr.hpp>

struct nested_t { char c; };
struct foo_t { int i; nested_t nested; };

static_assert(std::is_same<
    boost::pfr::flat_tuple_element_t<1, foo_t>, // Flat reflection.
    char    // `char`, not `nested_t`!
>::value, "");

// Requires C++17 or Loophole enabled:
static_assert(std::is_same<
    boost::pfr::tuple_element_t<1, foo_t>,      // Precise reflection.
    nested_t
>::value, "");

In other words, a flat reflection approach unwraps the nested structures, while the precise one accesses the fields as-is. In the example I've provided, the flattened structure will look like this:

struct nested_t { char c; };
struct foo_t { 
    int i;
    char c; // from nested_t 
};

Exactly the same will happen to your code:

struct foo { 
    int i; 
    int i_nested;
    short data_0;
    short data_1;
    short data_2;
};

[apolukhin]

During the Boost Formal Review a bunch of changes were requested and applied., including the removal of flat_ functions. That removes a lot of confusion.

I'll release a 2.0.0 version of the library soon. That version will be in Boost too.