https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90700
Bug ID: 90700
Summary: Wrong constraints for tuple(allocator_arg_t, const A&,
const tuple<UTypes...>&)
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
This should compile:
#include <tuple>
#include <memory>
struct X { };
struct Y
{
Y(const std::tuple<X>&) = delete;
Y(std::tuple<X>&&) { throw 1; }
Y(const X&) { }
};
int main()
{
const std::allocator<int> a;
std::tuple<X> x;
static_assert(!std::is_convertible<const std::tuple<X>&, Y>::value, "");
static_assert(!std::is_constructible<Y, const std::tuple<X>&>::value, "");
static_assert(!std::is_same<Y, X>::value, "");
// should use tuple<Y>::tuple<X>(allocator_arg_t, const A&, const tuple<X>&)
// and construct Y from X:
std::tuple<Y> y(std::allocator_arg, a, x);
}
tup.cc: In function 'int main()':
tup.cc:23:43: error: call of overloaded 'tuple(const std::allocator_arg_t&,
const std::allocator<int>&, std::tuple<X>&)' is ambiguous
23 | std::tuple<Y> y(std::allocator_arg, a, x);
| ^
In file included from tup.cc:1:
/home/jwakely/gcc/9/include/c++/9.1.1/tuple:751:2: note: candidate:
'std::tuple<_Elements>::tuple(std::allocator_arg_t, const _Alloc&,
std::tuple<_Elements>&&) [with _Alloc = std::allocator<int>; _Elements = {Y}]'
751 | tuple(allocator_arg_t __tag, const _Alloc& __a, tuple&& __in)
| ^~~~~
/home/jwakely/gcc/9/include/c++/9.1.1/tuple:747:2: note: candidate:
'std::tuple<_Elements>::tuple(std::allocator_arg_t, const _Alloc&, const
std::tuple<_Elements>&) [with _Alloc = std::allocator<int>; _Elements = {Y}]'
747 | tuple(allocator_arg_t __tag, const _Alloc& __a, const tuple& __in)
| ^~~~~
/home/jwakely/gcc/9/include/c++/9.1.1/tuple:709:2: note: candidate:
'std::tuple<_Elements>::tuple(std::allocator_arg_t, const _Alloc&, const
_Elements& ...) [with _Alloc = std::allocator<int>; _Dummy = void; typename
std::enable_if<(std::_TC<std::is_same<_Dummy, void>::value, _Elements
...>::_ConstructibleTuple<_Elements ...>() && std::_TC<std::is_same<_Dummy,
void>::value, _Elements ...>::_ImplicitlyConvertibleTuple<_Elements ...>()),
bool>::type <anonymous> = true; _Elements = {Y}]'
709 | tuple(allocator_arg_t __tag, const _Alloc& __a,
| ^~~~~
tup.cc:23:17: warning: unused variable 'y' [-Wunused-variable]
23 | std::tuple<Y> y(std::allocator_arg, a, x);
| ^
The problem is that the wrong value category is used in the constraints for the
tuple(allocator_arg_t, const A&, const tuple<UTypes...>&) constructor:
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -774,7 +774,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
&& _TMCT<_UElements...>::template
_ImplicitlyConvertibleTuple<_UElements...>()
&& _TNTC<_Dummy>::template
- _NonNestedTuple<tuple<_UElements...>&&>(),
+ _NonNestedTuple<const tuple<_UElements...>&>(),
bool>::type=true>
tuple(allocator_arg_t __tag, const _Alloc& __a,
const tuple<_UElements...>& __in)
@@ -789,7 +789,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
&& !_TMCT<_UElements...>::template
_ImplicitlyConvertibleTuple<_UElements...>()
&& _TNTC<_Dummy>::template
- _NonNestedTuple<tuple<_UElements...>&&>(),
+ _NonNestedTuple<const tuple<_UElements...>&>(),
bool>::type=false>
explicit tuple(allocator_arg_t __tag, const _Alloc& __a,
const tuple<_UElements...>& __in)
