Investigate and report ICE

[apolukhin]

test-2 and test-3: https://github.com/madmongo1/pfr_review/blob/master/pre-cxx20/test-2.cpp Seems to produce infinite recursive template expansion

The example should not compile, because the nested types are not aggregates.

In C++14 it gives a correct static assert

../../../boost/pfr/detail/core14_classic.hpp:509:5: error: static_assert failed "====================> Boost.PFR: Type can not be used is flat_ functions, because it's not POD"

in C++17 it also gives a correct static assert

../../../boost/pfr/detail/fields_count.hpp:225:9: error: static assertion failed: ====================> Boost.PFR: Type must be aggregate initializable.

Clang crashes after that. GCC goes into an infinite loop, ignoring all the template instantiation depth limitations and static_assert messages printing.

Using BOOST_PFR_PRECISE_FUNCTIONS_FOR now gives a complaint of missing sd::hash specialization:

../../../boost/pfr/detail/functional.hpp:133:9: error: static_assert failed due to requirement 'sizeof(boost::basic_string_view<char, std::char_traits >) && false' "====================> Boost.PFR: std::hash not specialized for type T"

I've added the tests and will report issues to the compiler developers soon.

[apolukhin]

Clang crush was fixes somewhere between clang-[10, 12]

GCC-9 issue could be reproduced via boost/libs/pfr/test$ ../../../b2 -j3 -a toolset=gcc-9 cxxstd=14 motivating_example0_classic