1 // RUN: %clang_cc1 -std=c++17 %s -verify
2 // expected-no-diagnostics
3 
4 // This test attempts to ensure that the below template parameter pack
5 // splitting technique executes in linear time in the number of template
6 // parameters. The size of the list below is selected so as to execute
7 // relatively quickly on a "good" compiler and to time out otherwise.
8 
9 template<typename...> struct TypeList;
10 
11 namespace detail {
12 template<unsigned> using Unsigned = unsigned;
13 template<typename T, T ...N> using ListOfNUnsignedsImpl = TypeList<Unsigned<N>...>;
14 template<unsigned N> using ListOfNUnsigneds =
15   __make_integer_seq<ListOfNUnsignedsImpl, unsigned, N>;
16 
17 template<typename T> struct TypeWrapper {
18   template<unsigned> using AsTemplate = T;
19 };
20 
21 template<typename ...N> struct Splitter {
22   template<template<N> class ...L,
23            template<unsigned> class ...R> struct Split {
24     using Left = TypeList<L<0>...>;
25     using Right = TypeList<R<0>...>;
26   };
27 };
28 }
29 
30 template<typename TypeList, unsigned N, typename = detail::ListOfNUnsigneds<N>>
31 struct SplitAtIndex;
32 
33 template<typename ...T, unsigned N, typename ...NUnsigneds>
34 struct SplitAtIndex<TypeList<T...>, N, TypeList<NUnsigneds...>> :
35   detail::Splitter<NUnsigneds...>::
36     template Split<detail::TypeWrapper<T>::template AsTemplate...> {};
37 
38 template<typename T, int N> struct Rep : Rep<typename Rep<T, N-1>::type, 1> {};
39 template<typename ...T> struct Rep<TypeList<T...>, 1> { typedef TypeList<T..., T...> type; };
40 
41 using Ints = Rep<TypeList<int>, 14>::type;
42 
43 template<typename T> extern int Size;
44 template<typename ...T> constexpr int Size<TypeList<T...>> = sizeof...(T);
45 
46 using Left = SplitAtIndex<Ints, Size<Ints> / 2>::Left;
47 using Right = SplitAtIndex<Ints, Size<Ints> / 2>::Right;
48 static_assert(Size<Left> == 8192);
49 static_assert(Size<Right> == 8192);
50 
51 template<typename L, typename R> struct Concat;
52 template<typename ...L, typename ...R> struct Concat<TypeList<L...>, TypeList<R...>> {
53   using type = TypeList<L..., R...>;
54 };
55 
56 using Ints = Concat<Left, Right>::type;
57