https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113854
Bug ID: 113854
Summary: the expression 'is_invocable_v ... evaluated to
'false'
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: f.heckenb...@fh-soft.de
Target Milestone: ---
The following error output is less than helpful. It contains a long litany of
mostly library internals. The ultimate error is "the expression
'is_invocable_v<_Fn, _Args ...> [with _Fn = main::._anon_116&; _Args =
{std::unique_ptr<int, std::default_delete<int> >&}]' evaluated to 'false'"
which, while technically correct, is still not very helpful.
The actual error is trying to pass a move-only type by value rather than by
reference, but I can't infer this from the messages. So again, unfortunately, I
have to read gcc's message as "something's wrong, but I won't tell you what"
...
One of the promises of concepts was better error messages. That's obviously not
the case here, compared to the version with plain old std::find_if (below).
% cat test.cpp
#include <vector>
#include <memory>
#include <ranges>
int main ()
{
std::vector <std::unique_ptr <int>> v;
std::ranges::find_if (v, [] (auto i) { return !!i; });
}
% g++ -std=c++20 test.cpp
test.cpp: In function 'int main()':
test.cpp:8:24: error: no match for call to '(const std::ranges::__find_if_fn)
(std::vector<std::unique_ptr<int> >&, main()::<lambda(auto:20)>)'
8 | std::ranges::find_if (v, [] (auto i) { return !!i; });
| ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/12/ranges:47,
from test.cpp:3:
/usr/include/c++/12/bits/ranges_util.h:474:7: note: candidate: 'template<class
_Iter, class _Sent, class _Proj, class _Pred> requires (input_iterator<_Iter>)
&& (sentinel_for<_Sent, _Iter>) && (indirect_unary_predicate<_Pred,
std::projected<_Iter, _Proj> >) constexpr _Iter
std::ranges::__find_if_fn::operator()(_Iter, _Sent, _Pred, _Proj) const'
474 | operator()(_Iter __first, _Sent __last,
| ^~~~~~~~
/usr/include/c++/12/bits/ranges_util.h:474:7: note: template argument
deduction/substitution failed:
test.cpp:8:24: note: candidate expects 4 arguments, 2 provided
8 | std::ranges::find_if (v, [] (auto i) { return !!i; });
| ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/12/bits/ranges_util.h:487:7: note: candidate: 'template<class
_Range, class _Proj, class _Pred> requires (input_range<_Range>) &&
(indirect_unary_predicate<_Pred,
std::projected<decltype(std::ranges::__cust_access::__begin((declval<_Container&>)())),
_Proj> >) constexpr std::ranges::borrowed_iterator_t<_Range>
std::ranges::__find_if_fn::operator()(_Range&&, _Pred, _Proj) const'
487 | operator()(_Range&& __r, _Pred __pred, _Proj __proj = {}) const
| ^~~~~~~~
/usr/include/c++/12/bits/ranges_util.h:487:7: note: template argument
deduction/substitution failed:
/usr/include/c++/12/bits/ranges_util.h:487:7: note: constraints not satisfied
In file included from /usr/include/c++/12/compare:39,
from /usr/include/c++/12/bits/stl_pair.h:65,
from /usr/include/c++/12/bits/stl_algobase.h:64,
from /usr/include/c++/12/vector:60,
from test.cpp:1:
/usr/include/c++/12/concepts: In substitution of 'template<class _Range, class
_Proj, class _Pred> requires (input_range<_Range>) &&
(indirect_unary_predicate<_Pred,
std::projected<decltype(std::ranges::__cust_access::__begin((declval<_Container&>)())),
_Proj> >) constexpr std::ranges::borrowed_iterator_t<_Range>
std::ranges::__find_if_fn::operator()(_Range&&, _Pred, _Proj) const [with
_Range = std::vector<std::unique_ptr<int> >&; _Proj = std::identity; _Pred =
main()::<lambda(auto:20)>]':
test.cpp:8:24: required from here
/usr/include/c++/12/concepts:336:13: required for the satisfaction of
'invocable<_Fn, _Args ...>' [with _Fn = main::._anon_116&; _Args =
{std::unique_ptr<int, std::default_delete<int> >&}]
/usr/include/c++/12/concepts:340:13: required for the satisfaction of
'regular_invocable<_Fn, _Args ...>' [with _Fn = main::._anon_116&; _Args =
{std::unique_ptr<int, std::default_delete<int> >&}]
/usr/include/c++/12/concepts:344:13: required for the satisfaction of
'predicate<_Fn&, typename std::__detail::__iter_traits_impl<typename
std::remove_cvref<_Tp2>::type, std::indirectly_readable_traits<typename
std::remove_cvref<_Tp2>::type> >::type::value_type&>' [with _Fn =
main::._anon_116; _Iter =
std::projected<__gnu_cxx::__normal_iterator<std::unique_ptr<int,
std::default_delete<int> >*, std::vector<std::unique_ptr<int,
std::default_delete<int> >, std::allocator<std::unique_ptr<int,
std::default_delete<int> > > > >, std::identity>]
/usr/include/c++/12/bits/iterator_concepts.h:710:13: required for the
satisfaction of 'indirect_unary_predicate<_Pred, std::projected<decltype
(std::ranges::__cust_access::__begin(declval<_Container&>())), _Proj> >' [with
_Pred = main::._anon_116; _Range = std::vector<std::unique_ptr<int,
std::default_delete<int> >, std::allocator<std::unique_ptr<int,
std::default_delete<int> > > >&; _Proj = std::identity]
/usr/include/c++/12/concepts:336:25: note: the expression 'is_invocable_v<_Fn,
_Args ...> [with _Fn = main::._anon_116&; _Args = {std::unique_ptr<int,
std::default_delete<int> >&}]' evaluated to 'false'
336 | concept invocable = is_invocable_v<_Fn, _Args...>;
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
% cat test2.cpp
#include <vector>
#include <memory>
#include <algorithm>
int main ()
{
std::vector <std::unique_ptr <int>> v;
std::find_if (v.begin (), v.end (), [] (auto i) { return !!i; });
}
% g++ -std=c++20 test2.cpp
In file included from /usr/include/c++/12/bits/stl_algobase.h:71,
from /usr/include/c++/12/vector:60,
from test2.cpp:1:
/usr/include/c++/12/bits/predefined_ops.h: In instantiation of 'constexpr bool
__gnu_cxx::__ops::_Iter_pred<_Predicate>::operator()(_Iterator) [with _Iterator
= __gnu_cxx::__normal_iterator<std::unique_ptr<int>*,
std::vector<std::unique_ptr<int> > >; _Predicate = main()::<lambda(auto:19)>]':
/usr/include/c++/12/bits/stl_algobase.h:2067:14: required from 'constexpr
_RandomAccessIterator std::__find_if(_RandomAccessIterator,
_RandomAccessIterator, _Predicate, random_access_iterator_tag) [with
_RandomAccessIterator = __gnu_cxx::__normal_iterator<unique_ptr<int>*,
vector<unique_ptr<int> > >; _Predicate =
__gnu_cxx::__ops::_Iter_pred<main()::<lambda(auto:19)> >]'
/usr/include/c++/12/bits/stl_algobase.h:2112:23: required from 'constexpr
_Iterator std::__find_if(_Iterator, _Iterator, _Predicate) [with _Iterator =
__gnu_cxx::__normal_iterator<unique_ptr<int>*, vector<unique_ptr<int> > >;
_Predicate = __gnu_cxx::__ops::_Iter_pred<main()::<lambda(auto:19)> >]'
/usr/include/c++/12/bits/stl_algo.h:3877:28: required from 'constexpr _IIter
std::find_if(_IIter, _IIter, _Predicate) [with _IIter =
__gnu_cxx::__normal_iterator<unique_ptr<int>*, vector<unique_ptr<int> > >;
_Predicate = main()::<lambda(auto:19)>]'
test2.cpp:8:16: required from here
/usr/include/c++/12/bits/predefined_ops.h:318:30: error: use of deleted
function 'std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp,
_Dp>&) [with _Tp = int; _Dp = std::default_delete<int>]'
318 | { return bool(_M_pred(*__it)); }
| ~~~~~~~^~~~~~~
In file included from /usr/include/c++/12/memory:76,
from test2.cpp:2:
/usr/include/c++/12/bits/unique_ptr.h:514:7: note: declared here
514 | unique_ptr(const unique_ptr&) = delete;
| ^~~~~~~~~~
test2.cpp:8:39: note: initializing argument 1 of 'main()::<lambda(auto:19)>
[with auto:19 = std::unique_ptr<int>]'
8 | std::find_if (v.begin (), v.end (), [] (auto i) { return !!i; });
| ^
