brigand::transform causes GCC and clang to never finish

[jcelerier]

Repro :

#include <brigand/algorithms/transform.hpp>

template<typename... Args>
struct typelist { };

template <typename T>
struct get_type
{
  using type = typename T::type;
};

struct foobar
{
    static const constexpr typelist<> empty{};
};

int main()
{
    using transformed = brigand::transform<decltype(foobar::empty), get_type<brigand::_1>>;
    transformed t;
    return 0;
}

for me, with gcc-9.2 and clang-9, building never terminates (actually, it munches through my 32 gigabytes of RAM in ~1 minute and kills my machine).
It works when replacing e.g. decltype(foobar::empty) with typelist<> directly.

Any idea of where that could come from ?

[jonathanpoelen]

decltype(foobar::empty) = const typelist<>. internally, this builds a list of -1 items (i.e. 18446744073709551615 :D)

Brigand is not designed to work with a constant typelist (nor with volatile or reference). You will need to use std::remove_const_t/std::decay_t.

[jcelerier]

ah, indeed, back<list<>> was the culprit.

would it not be possible to add a partial specialization which handles the cv/ref-qualified empty list case ?

[jfalcou]

Hi,

This is indeed an oversight. We need to decide how to report error : static_assert or be SFINAE friendly and then we can add it.