1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
|
// PR c++/91966
// { dg-do compile { target c++11 } }
// 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() {}
|
