https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91966
Bug ID: 91966
Summary: pack expansion for Cartesian product breaks if
certain indirections are involved
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ecrypa at posteo dot de
Target Milestone: ---
The following metaprogramming produces an unexpected output: see the types
`computed` and `expected` at the end.
https://godbolt.org/z/pX7WxD
BEGIN_CODE
// Reduced to this include-free example. Further reduction is hard: Either
// the bug(?) disappears, or the program becomes meaningless.
template<class...>
struct list {};
struct nil;
////////////////////////////////////////////////////////////////////////////////
template<int n>
struct number {
constexpr /*implicit*/ operator int() const { return n; }
using type = number<n>;
};
using false_ = number<0>;
using true_ = number<1>;
static_assert(!false_{}, "");
static_assert(true_{}, "");
template<int... ns> using numbers = list<number<ns>...>;
////////////////////////////////////////////////////////////////////////////////
template<class lhs, class rhs>
struct less_impl;
template<int lhs, int rhs>
struct less_impl<number<lhs>, number<rhs>>
: number<(lhs < rhs)> {};
template<class lhs, class rhs> using less = typename less_impl<lhs, rhs>::type;
////////////////////////////////////////////////////////////////////////////////
template<class v0, class... vs>
struct sum_impl {
static_assert(sizeof...(vs) == 0, "see specialization");
using type = v0;
};
template<int v0, int v1, class... vs>
struct sum_impl<number<v0>, number<v1>, vs...>
: sum_impl<number<v0 + v1>, vs...> {};
template<class... nums> using sum = typename sum_impl<nums...>::type;
////////////////////////////////////////////////////////////////////////////////
template<class num>
struct conditional_impl {
static_assert(num{}, "see specialization");
template<class T, class F>
using type = T;
};
template<>
struct conditional_impl<false_> {
template<class T, class F>
using type = F;
};
template<class num, class T, class F>
using conditional = typename conditional_impl<num>::template type<T, F>;
////////////////////////////////////////////////////////////////////////////////
template<class seq>
struct min_filter_impl;
template<class... nums>
struct min_filter_impl<list<nums...>> {
template<class num>
using count_better_mins = sum<less<nums, num>...>;
using type = list<conditional<count_better_mins<nums>, nil, nums>...>;
//// using debug = list<conditional<count_better_mins<nums>, nil, void>...>;
// error: expansion pattern 'conditional<typename sum_impl<less<nums,
nums>...>::type, nil, void>' contains no parameter packs
};
template<class seq> using min_filter = typename min_filter_impl<seq>::type;
////////////////////////////////////////////////////////////////////////////////
void test_min_filter() {
using computed = min_filter<numbers<2, 7, 2>>;
using expected = list<number<2>, nil, number<2>>;
(void)(computed{} = expected{});// compiles for identical types
// error: no match for 'operator=' (operand types are 'computed' {aka
'list<number<2>, number<7>, number<2> >'} and 'expected' {aka 'list<number<2>,
nil, number<2> >'})
}
int main() {}
END_CODE
