Using trailing comma to describe enum yields unscrutable errors

[ddevienne]

My enums are code-generated, and the last enumerator has a trailing comma, which is OK in C++11.
But when I copy/pasted it to turn it into a BOOST_DESCRIBE_ENUM, this yielding weird errors like below.
Removing the trailing comma fixed the problem, but I wasted quite a bit of time on this.

Any way to fix this, so it works with trailing commas, as enums do?
Or at least tweak the code to yield a better error message?

1>DescribeTests.cpp
1>D:\boost/describe/enum.hpp(29,1): error C2182: 'value': illegal use of type 'void'
1>D:\boost\1.74.0\Win_x64_10_v16_debug\include\boost/mp11/algorithm.hpp(1037): message : see reference to class template instantiation 'boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_9250c7397ae982f1d2ae378dbdd22917>::()::D>' being compiled
1>D:\boost\1.74.0\Win_x64_10_v16_debug\include\boost/mp11/algorithm.hpp(1055): message : see reference to function template instantiation 'F boost::mp11::detail::mp_for_each_impl<boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_22d39ba0fa5275074785db73206bfcec>::()::D>,boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_b36e895947c870ba551a0beff49d6047>::()::D>,boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_294ffb3a79474efc9e18b549fb7261d1>::()::D>,boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_9250c7397ae982f1d2ae378dbdd22917>::()::D>,_Ty>(boost::mp11::mp_list<boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_22d39ba0fa5275074785db73206bfcec>::()::D>,boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_b36e895947c870ba551a0beff49d6047>::()::D>,boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_294ffb3a79474efc9e18b549fb7261d1>::()::D>,boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_9250c7397ae982f1d2ae378dbdd22917>::()::D>>,F &&)' being compiled
1>        with
1>        [
1>            F=`anonymous-namespace'::enum_to_string::<lambda_35eeefbe1e328ec36c865ccd2344c156>,
1>            _Ty=`anonymous-namespace'::enum_to_string::<lambda_35eeefbe1e328ec36c865ccd2344c156>
1>        ]
1>D:\pdgm\trunk\psc3\SharedComponents\src\tests\eml\DescribeTests.cpp(26): message : see reference to function template instantiation '`anonymous-namespace'::enum_to_string::<lambda_35eeefbe1e328ec36c865ccd2344c156> boost::mp11::mp_for_each<boost::describe::detail::list<boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_22d39ba0fa5275074785db73206bfcec>::()::D>,boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_b36e895947c870ba551a0beff49d6047>::()::D>,boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_294ffb3a79474efc9e18b549fb7261d1>::()::D>,boost::describe::detail::enum_descriptor<Foo::_enum_descriptor_fn::<lambda_9250c7397ae982f1d2ae378dbdd22917>::()::D>>,`anonymous-namespace'::enum_to_string::<lambda_35eeefbe1e328ec36c865ccd2344c156>>(F &&)' being compiled
1>        with
1>        [
1>            F=`anonymous-namespace'::enum_to_string::<lambda_35eeefbe1e328ec36c865ccd2344c156>
1>        ]
1>D:\pdgm\trunk\psc3\SharedComponents\src\tests\eml\DescribeTests.cpp(42): message : see reference to function template instantiation 'const char *`anonymous-namespace'::enum_to_string<Foo::Bar>(E)' being compiled
1>        with
1>        [
1>            E=Foo::Bar
1>        ]
1>D:\boost/describe/enum.hpp(29,1): error C2440: 'initializing': cannot convert from 'void' to 'const int'
1>D:\boost/describe/enum.hpp(29,59): message : Expressions of type void cannot be converted to other types
1>Done building project "tests_describe.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

[pdimov]

Can you please post the exact code?

[ddevienne]

I suspect adding a trailing comma to your own example would be enough, but here goes anyway.
I tried a bunch of things before realizing the trailing-comma mistake I made, w/ or w/o namespaces,
separate TUs for the def and describe or not (former is my use case), longer names, etc... Thus the weird code.

PS: it's not the same exact code I sent errors for, but fails the same way
PPS: the test builds and passes when removing the trailing comma after eCcdEfghIjklmop on BOOST_DESCRIBE_ENUM

#include <boost/describe.hpp>
#include <boost/mp11.hpp>
#include <boost/test/unit_test.hpp>

namespace Foo {

enum Bar { eAbcdEfghIjklm, eBcdEfghIjklmo, eCcdEfghIjklmop, };

BOOST_DESCRIBE_ENUM(Bar, eAbcdEfghIjklm, eBcdEfghIjklmo, eCcdEfghIjklmop,)

}

namespace {

template<class E> char const * enum_to_string(E e) {
    const char* r = "(unnamed)";

    boost::mp11::mp_for_each<boost::describe::describe_enumerators<E>>(
        [&](auto D) {
            if (e == D.value) {
                r = D.name;
            }
        }
    );
    return r;
}

BOOST_AUTO_TEST_SUITE(DescribeTests)

BOOST_AUTO_TEST_CASE(test_enum) {
    using namespace Foo;
    Bar bar = eCcdEfghIjklmop;
    std::string bar_name = enum_to_string(bar);
    BOOST_CHECK_EQUAL(bar_name, "eCcdEfghIjklmop");
}

BOOST_AUTO_TEST_SUITE_END()

}

Microsoft Visual Studio Professional 2019, Version 16.7.6
Visual C++ 2019 00435-30010-36205-AA935
In C++17 mode.

[ddevienne]

Oh, forgot to mention, I layered your Describe lib on top of Boost 1.74, which is my company's current version I can't upgrade.
Doesn't seem to give any trouble. Please keep this in mind, we can't all upgrade to the latest and greatest, yet still want to use
your great libraries Peter :)

[pdimov]

A trailing comma should now work (although it does cause a preprocessor warning on MSVC.)