[Bug c++/108414] Incorrect pack expansion as argument to lambda

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108414

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-01-16
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
            Summary|template meta programming   |Incorrect pack expansion as
                   |                            |argument to lambda

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This works with or without -DUSING when -DNAMED_FUNCTION is used:

using size_t = decltype(sizeof(0));
template<typename T> struct type_identity { using type = T; };
template<typename T, typename U> constexpr bool is_same_v = false;
template<typename T> constexpr bool is_same_v<T, T> = true;

template<typename... Ts>
struct type_list {};

template <size_t I, typename T>
struct indexed { using type = T; };

template <size_t I, typename ...Ts>
struct indexer;

template <size_t I, typename T, typename ...Ts>
struct indexer<I, T, Ts...> : indexed<I, T>, indexer<I+1, Ts...> {};

template <size_t I, typename T>
struct indexer<I, T> : indexed<I, T> {};

#ifndef NAMED_FUNCTION
template <size_t I, typename ...Ts>
using nth_element_t = typename decltype([]<typename T>(const indexed<I,
T>&){return type_identity<T>{};}(indexer<0, Ts...>{}))::type;
#else
template<size_t I, typename T> auto select(const indexed<I, T>&) { return
type_identity<T>{}; }
template <size_t I, typename ...Ts>
using nth_element_t = typename decltype(select<I>(indexer<0, Ts...>{}))::type;
#endif

template<size_t I, typename T>
struct nth_type;

template<size_t I, typename ... Ts>
struct nth_type< I, type_list<Ts...> >
{
#ifndef USING
  typedef nth_element_t<I, Ts...> type; // Incorrect result with it
#else
  using type = nth_element_t<I, Ts...>; // Compile error
#endif
};

template<size_t I, typename T>
using nth_type_t = typename nth_type<I, T>::type;

using list = type_list<int, float, double>;

using elm0 = nth_type_t<0, list>;
using elm1 = nth_type_t<1, list>;
using elm2 = nth_type_t<2, list>;

static_assert( is_same_v<elm0, int> );
static_assert( is_same_v<elm1, float> );
static_assert( is_same_v<elm2, double> );

So as Andrew said, the problem is the lambda in the decltype expression.