Exponential compilation times with simple typelist implementation. Why?

using type = typename std::conditional<
      // Does the predicate hold on the head of the input list?
      P<Head>::value,
      // The head of the input list matches our predictate, copy it
      typename filter_tl_impl<tl<Tail...>, tl<Ts2..., Head>, P>::type,
      // The head of the input list does not match our predicate, skip
      // it
      typename filter_tl_impl<tl<Tail...>, tl<Ts2...>, P>::type>::type;

instantiates both sides because of ::type.

You might delay intermediate instantiation by after the std::conditional:

using type = typename std::conditional<
      // Does the predicate hold on the head of the input list?
      P<Head>::value,
      // The head of the input list matches our predicate, copy it
      filter_tl_impl<tl<Tail...>, tl<Ts2..., Head>, P>,
      // The head of the input list does not match our predicate, skip
      // it
      filter_tl_impl<tl<Tail...>, tl<Ts2...>, P>>::type::type;

Which leads to linear number of instantiations instead of exponential.


If you want lists, the first thing you do is define the cons function. The rest becomes natural and straightforward.

// first, define `cons`      
  template <class Head, class T> struct cons_impl;
  template <class Head, class ... Tail>
  struct cons_impl <Head, tl<Tail...>> {
     using type = tl<Head, Tail...>;
  };
  template <class Head, class T>
  using cons = typename cons_impl<Head, T>::type;

// next, define `filter`
  template <template <typename> class P, class T>
  struct filter_tl_impl;
  template <template <typename> class P, class T>
  using filter_tl = typename filter_tl_impl<P, T>::type;

// empty list case      
  template <template <typename> class P>
  struct filter_tl_impl<P, tl<>> {
    using type = tl<>;
  };
  
// non-empty lust case
  template <template <typename> class P, class Head, class ... Tail>
  struct filter_tl_impl<P, tl<Head, Tail...>> {
    using tailRes = filter_tl<P, tl<Tail...>>;
    using type = std::conditional_t<P<Head>::value,
                                    cons<Head, tailRes>,
                                    tailRes>;
  };

Note tailRes is defined for readability only, you can write directly

    using type = std::conditional_t<P<Head>::value,
                                    cons<Head, filter_tl<P, tl<Tail...>>>,
                                    filter_tl<P, tl<Tail...>>>;

and compilation time remains negligible.

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