[lwt] `BOOST_TEST_TRAIT` fails to compile if the trait involves an incomplete type

[Lastique]

Here's an example:

#include <boost/core/lightweight_test_trait.hpp>

template< typename T >
struct trait
{
    static constexpr bool value = false;
};

struct incomplete;

int main()
{
    BOOST_TEST_TRAIT_FALSE((trait< incomplete >));
    return boost::report_errors();
}

In file included from /opt/compiler-explorer/libs/boost_1_82_0/boost/core/lightweight_test_trait.hpp:24,
                 from <source>:1:
/opt/compiler-explorer/libs/boost_1_82_0/boost/core/type_name.hpp: In instantiation of 'std::string boost::core::detail::typeid_name() [with T = incomplete; std::string = std::__cxx11::basic_string<char>]':
/opt/compiler-explorer/libs/boost_1_82_0/boost/core/type_name.hpp:214:30:   required from 'static std::string boost::core::detail::tn_holder<T>::type_name(const std::string&) [with T = incomplete; std::string = std::__cxx11::basic_string<char>]'
/opt/compiler-explorer/libs/boost_1_82_0/boost/core/type_name.hpp:499:34:   required from 'int boost::core::detail::tn_add_each_impl(std::string&) [with T = incomplete; std::string = std::__cxx11::basic_string<char>]'
/opt/compiler-explorer/libs/boost_1_82_0/boost/core/type_name.hpp:508:36:   required from 'std::string boost::core::detail::tn_add_each() [with T = {incomplete}; std::string = std::__cxx11::basic_string<char>]'
/opt/compiler-explorer/libs/boost_1_82_0/boost/core/type_name.hpp:1057:43:   required from 'static std::string boost::core::detail::tn_holder<L<T ...> >::type_name(const std::string&) [with L = trait; T = {incomplete}; std::string = std::__cxx11::basic_string<char>]'
/opt/compiler-explorer/libs/boost_1_82_0/boost/core/type_name.hpp:1159:49:   required from 'std::string boost::core::type_name() [with T = trait<incomplete>; std::string = std::__cxx11::basic_string<char>]'
/opt/compiler-explorer/libs/boost_1_82_0/boost/core/lightweight_test_trait.hpp:44:41:   required from 'void boost::detail::test_trait_impl(const char*, void (*)(T), bool, const char*, int, const char*) [with T = trait<incomplete>]'
<source>:13:5:   required from here
/opt/compiler-explorer/libs/boost_1_82_0/boost/core/type_name.hpp:107:29: error: invalid use of incomplete type 'struct incomplete'
  107 |     return fix_typeid_name( typeid(T).name() );
      |                             ^~~~~~~~~
<source>:9:8: note: forward declaration of 'struct incomplete'
    9 | struct incomplete;
      |        ^~~~~~~~~~

The test intentionally uses an incomplete type to test the trait's behavior wrt. incomplete types.

[Lastique]

Actually, there are problems with typeid besides the incomplete types, see here, for example. Looks like boost::core::type_name() is broken for member function types as well.

[Lastique]

Why do we need boost::core::type_name again? I thought demangle would be enough.

[pdimov]

Looks like typeid doesn't work on qualified member functions, but only under C++03 and for Clang after 10.

[pdimov]

See https://godbolt.org/z/M5s348jxf for example. Under MSVC, it also cleans up things like struct X, __cdecl, __ptr64 and other cruft.

[Lastique]

See https://godbolt.org/z/M5s348jxf for example. Under MSVC, it also cleans up things like struct X, __cdecl, __ptr64 and other cruft.

This could be achieved by string modification instead of decomposing types and generating type_info structures for each component.

[pdimov]

Both cases should be fixed on develop.

This could be achieved by string modification instead of decomposing types and generating type_info structures for each component.

Maybe... but I don't think it would be particularly easy. There are a lot of other weird differences between the demangled typeid(T).name() such as whitespace placement, [] being [0], unsigned long long being unsigned __int64, etc, and figuring out std::map<std::string, std::string> would basically require parsing the string back into a type structure, or doing creative things with regex capture matching. type_name takes care of all that. It also works with -fno-rtti and can do slightly better than just giving up.

In addition, there are odd cases where typeid(T).name() doesn't work, such as incomplete types (fixed now thanks to you) and e.g. std::pair<void, void>.

All this is admittedly overkill for the purposes of BOOST_TEST_TRAIT, looking at its output was just what motivated me to write type_name.

[Lastique]

Both cases should be fixed on develop.

Thank you. Although I have already switched to BOOST_TEST in these test cases.