https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102777
Bug ID: 102777
Summary: Error when requires expressions with parameter pack
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ispavlick at gmail dot com
Target Milestone: ---
// clang compiles without errors.
#include <tuple>
#include <type_traits>
#include <iostream>
using namespace std;
template <typename T>
constexpr bool check_visitor = []<size_t ...I>(index_sequence<I...>){
return ((requires (tuple_element_t<I, T> fun)
{{fun(3)}->same_as<void>;}) && ...);
}(make_index_sequence<tuple_size_v<T>>{});
int main() {
auto f1 = [](int) {};
auto f2 = [](int) {};
static_assert(check_visitor<tuple<decltype(f1), decltype(f2)>>);
}
$ g++ -std=c++20 1.cpp
1.cpp: In lambda function:
1.cpp:9:22: error: operand of fold expression has no unexpanded parameter packs
9 | return ((requires (tuple_element_t<I, T> fun)
| ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
10 | {{fun(3)}->same_as<void>;}) &&
...);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~
1.cpp: In instantiation of ‘constexpr const bool
check_visitor<std::tuple<main()::<lambda(int)>, main()::<lambda(int)> > >’:
1.cpp:16:16: required from here
1.cpp:11:2: error: void value not ignored as it ought to be
8 | constexpr bool check_visitor = []<size_t ...I>(index_sequence<I...>){
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
9 | return ((requires (tuple_element_t<I, T> fun)
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
10 | {{fun(3)}->same_as<void>;}) &&
...);
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
11 | }(make_index_sequence<tuple_size_v<T>>{});
| ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1.cpp: In function ‘int main()’:
1.cpp:16:23: error: non-constant condition for static assertion
16 | static_assert(check_visitor<tuple<decltype(f1),
decltype(f2)>>);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
