hctim added a comment.
Hi folks,
I found an issue with building Android using this patch. I've reduced it down
to the following problem where the evaluation of the `std::visit` is believed
to be non-exhaustive, but it seems like it is? Would you mind taking a look?
Admittedly, my knowledge in this area of the cxx stdlib is not so great.
Thanks.
$ cat /tmp/variant.cpp
#include <utility>
#include <variant>
struct A {};
struct B {};
using MyVariant = std::variant<A, B>;
// Helper to std::visit with lambdas.
template <typename... V>
struct Visitor : V... {};
// explicit deduction guide (not needed as of C++20)
template <typename... V>
Visitor(V...) -> Visitor<V...>;
const char* toString(const MyVariant& args) {
Visitor toStringVisitor{
[&](const A&) { return "A"; },
[&](const B&) { return "B"; },
};
return std::visit(toStringVisitor, args);
}
$ ~/llvm-build/opt/bin/clang++ /tmp/variant.cpp -c -o /tmp/variant.o
-std=c++17 -stdlib=libc++
In file included from /tmp/variant.cpp:2:
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:680:19:
error: static assertion failed due to requirement
'is_invocable_v<Visitor<(lambda at /tmp/variant.cpp:18:13), (lambda at
/tmp/variant.cpp:19:13)> &, const A &>': `std::visit` requires the visitor to
be exhaustive.
680 | static_assert(is_invocable_v<_Visitor, _Values...>,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:689:7:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__variant::__std_visit_exhaustive_visitor_check<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &, const A &>'
requested here
689 | __std_visit_exhaustive_visitor_check<
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/__type_traits/invoke.h:337:10:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)>
&>::operator()<const std::__variant_detail::__alt<0, A> &>' requested here
337 | decltype(std::declval<_Fp>()(std::declval<_Args>()...))
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:572:16:
note: while substituting deduced template arguments into function template
'__invoke' [with _Fp =
std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &>, _Args =
<const __alt<0UL, A> &>]
572 | return _VSTD::__invoke(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/__config:805:17:
note: expanded from macro '_VSTD'
805 | # define _VSTD std
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:581:43:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__dispatcher<0>::__dispatch<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &>' requested here
581 | return __dispatcher<_Is...>::template __dispatch<_Fp, _Vs...>;
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:608:12:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_dispatch<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &, 0UL>' requested here
608 | return __make_dispatch<_Fp, _Vs...>(__is);
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:616:34:
note: (skipping 2 contexts in backtrace; use -ftemplate-backtrace-limit=0 to
see all)
616 | return __base::__make_farray(__make_fmatrix_impl<_Fp, _Vs...>(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:533:9:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_fmatrix<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &>' requested here
533 | __make_fmatrix<_Visitor&&,
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:642:20:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__visit_alt<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &>, const
std::__variant_detail::__impl<A, B> &>' requested here
642 | return __base::__visit_alt(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:661:12:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__variant::__visit_alt<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &>, const
std::variant<A, B> &>' requested here
661 | return __visit_alt(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:1759:21:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__variant::__visit_value<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &, const
std::variant<A, B> &>' requested here
1759 | return __variant::__visit_value(_VSTD::forward<_Visitor>(__visitor),
| ^
/tmp/variant.cpp:21:17: note: in instantiation of function template
specialization 'std::visit<Visitor<(lambda at /tmp/variant.cpp:18:13), (lambda
at /tmp/variant.cpp:19:13)> &, const std::variant<A, B> &, void>' requested here
21 | return std::visit(toStringVisitor, args);
| ^
In file included from /tmp/variant.cpp:2:
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:692:14:
error: attempt to use a deleted function
692 | return _VSTD::__invoke(_VSTD::forward<_Visitor>(__visitor),
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/__config:805:17:
note: expanded from macro '_VSTD'
805 | # define _VSTD std
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/__type_traits/invoke.h:337:10:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)>
&>::operator()<const std::__variant_detail::__alt<0, A> &>' requested here
337 | decltype(std::declval<_Fp>()(std::declval<_Args>()...))
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:572:16:
note: while substituting deduced template arguments into function template
'__invoke' [with _Fp =
std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &>, _Args =
<const __alt<0UL, A> &>]
572 | return _VSTD::__invoke(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/__config:805:17:
note: expanded from macro '_VSTD'
805 | # define _VSTD std
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:581:43:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__dispatcher<0>::__dispatch<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &>' requested here
581 | return __dispatcher<_Is...>::template __dispatch<_Fp, _Vs...>;
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:608:12:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_dispatch<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &, 0UL>' requested here
608 | return __make_dispatch<_Fp, _Vs...>(__is);
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:616:34:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_fmatrix_impl<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &, 0UL>' requested here
616 | return __base::__make_farray(__make_fmatrix_impl<_Fp, _Vs...>(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:623:12:
note: (skipping 1 context in backtrace; use -ftemplate-backtrace-limit=0 to
see all)
623 | return __make_fmatrix_impl<_Fp, _Vs...>(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:533:9:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_fmatrix<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &>' requested here
533 | __make_fmatrix<_Visitor&&,
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:642:20:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__visit_alt<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &>, const
std::__variant_detail::__impl<A, B> &>' requested here
642 | return __base::__visit_alt(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:661:12:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__variant::__visit_alt<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &>, const
std::variant<A, B> &>' requested here
661 | return __visit_alt(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:1759:21:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__variant::__visit_value<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &, const
std::variant<A, B> &>' requested here
1759 | return __variant::__visit_value(_VSTD::forward<_Visitor>(__visitor),
| ^
/tmp/variant.cpp:21:17: note: in instantiation of function template
specialization 'std::visit<Visitor<(lambda at /tmp/variant.cpp:18:13), (lambda
at /tmp/variant.cpp:19:13)> &, const std::variant<A, B> &, void>' requested here
21 | return std::visit(toStringVisitor, args);
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/__type_traits/nat.h:25:3:
note: '~__nat' has been explicitly marked deleted here
25 | ~__nat() = delete;
| ^
In file included from /tmp/variant.cpp:2:
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:572:16:
error: attempt to use a deleted function
572 | return _VSTD::__invoke(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/__config:805:17:
note: expanded from macro '_VSTD'
805 | # define _VSTD std
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:581:43:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__dispatcher<0>::__dispatch<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &>' requested here
581 | return __dispatcher<_Is...>::template __dispatch<_Fp, _Vs...>;
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:608:12:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_dispatch<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &, 0UL>' requested here
608 | return __make_dispatch<_Fp, _Vs...>(__is);
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:616:34:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_fmatrix_impl<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &, 0UL>' requested here
616 | return __base::__make_farray(__make_fmatrix_impl<_Fp, _Vs...>(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:623:12:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_fmatrix_impl<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &, 0UL, 1UL>' requested here
623 | return __make_fmatrix_impl<_Fp, _Vs...>(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:533:9:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_fmatrix<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &>' requested here
533 | __make_fmatrix<_Visitor&&,
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:642:20:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__visit_alt<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &>, const
std::__variant_detail::__impl<A, B> &>' requested here
642 | return __base::__visit_alt(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:661:12:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__variant::__visit_alt<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &>, const
std::variant<A, B> &>' requested here
661 | return __visit_alt(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:1759:21:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__variant::__visit_value<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &, const
std::variant<A, B> &>' requested here
1759 | return __variant::__visit_value(_VSTD::forward<_Visitor>(__visitor),
| ^
/tmp/variant.cpp:21:17: note: in instantiation of function template
specialization 'std::visit<Visitor<(lambda at /tmp/variant.cpp:18:13), (lambda
at /tmp/variant.cpp:19:13)> &, const std::variant<A, B> &, void>' requested here
21 | return std::visit(toStringVisitor, args);
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/__type_traits/nat.h:25:3:
note: '~__nat' has been explicitly marked deleted here
25 | ~__nat() = delete;
| ^
In file included from /tmp/variant.cpp:2:
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:581:12:
error: cannot deduce return type 'auto' from returned value of type
'<overloaded function type>'
581 | return __dispatcher<_Is...>::template __dispatch<_Fp, _Vs...>;
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:608:12:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_dispatch<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &, 0UL>' requested here
608 | return __make_dispatch<_Fp, _Vs...>(__is);
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:616:34:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_fmatrix_impl<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &, 0UL>' requested here
616 | return __base::__make_farray(__make_fmatrix_impl<_Fp, _Vs...>(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:623:12:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_fmatrix_impl<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &, 0UL, 1UL>' requested here
623 | return __make_fmatrix_impl<_Fp, _Vs...>(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:533:9:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__make_fmatrix<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &> &&, const
std::__variant_detail::__base<std::__variant_detail::_Trait::_TriviallyAvailable,
A, B> &>' requested here
533 | __make_fmatrix<_Visitor&&,
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:642:20:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__base::__visit_alt<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &>, const
std::__variant_detail::__impl<A, B> &>' requested here
642 | return __base::__visit_alt(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:661:12:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__variant::__visit_alt<std::__variant_detail::__visitation::__variant::__value_visitor<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &>, const
std::variant<A, B> &>' requested here
661 | return __visit_alt(
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:1759:21:
note: in instantiation of function template specialization
'std::__variant_detail::__visitation::__variant::__visit_value<Visitor<(lambda
at /tmp/variant.cpp:18:13), (lambda at /tmp/variant.cpp:19:13)> &, const
std::variant<A, B> &>' requested here
1759 | return __variant::__visit_value(_VSTD::forward<_Visitor>(__visitor),
| ^
/tmp/variant.cpp:21:17: note: in instantiation of function template
specialization 'std::visit<Visitor<(lambda at /tmp/variant.cpp:18:13), (lambda
at /tmp/variant.cpp:19:13)> &, const std::variant<A, B> &, void>' requested here
21 | return std::visit(toStringVisitor, args);
| ^
/usr/local/google/home/mitchp/llvm-build/opt/bin/../include/c++/v1/variant:571:37:
note: candidate template ignored: failed template argument deduction
571 | static constexpr decltype(auto) __dispatch(_Fp __f, _Vs... __vs) {
| ^
4 errors generated.
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