http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60497
Bug ID: 60497
Summary: unique_ptr<T> tries to complete its type T even though
it's not required to be a complete type
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: richard-gccbugzilla at metafoo dot co.uk
Consider:
struct A;
template<typename T> struct B { T t; };
struct Deleter { void operator()(B<A>*) const; };
std::unique_ptr<B<A>, Deleter> u;
It's not *entirely* clear to me that this is valid, but I think it is intended
to be. unique_ptr is required to support incomplete types like B<A>, but it's
not obvious that it can't *try* to complete the type. (If it does, this won't
compile, because B<A> has a field of incomplete type 'A'.)
g++ 4.8 rejects this:
<stdin>: In instantiation of 'struct B<A>':
tuple:758:35: required from 'constexpr typename std::__add_ref<typename
std::tuple_element<__i, std::tuple<_Elements ...> >::type>::type
std::get(std::tuple<_Elements ...>&) [with long unsigned int __i = 0ul;
_Elements = {B<A>*, Deleter}; typename std::__add_ref<typename
std::tuple_element<__i, std::tuple<_Elements ...> >::type>::type = B<A>*&]'
bits/unique_ptr.h:182:32: required from 'std::unique_ptr<_Tp,
_Dp>::~unique_ptr() [with _Tp = B<A>; _Dp = Deleter]'
<stdin>:5:34: required from here
<stdin>:3:37: error: 'B<T>::t' has incomplete type
<stdin>:2:10: error: forward declaration of 'struct A'
This is ultimately a bug in std::tuple::get:
template<std::size_t __i, typename... _Elements>
constexpr typename __add_ref<
typename tuple_element<__i, tuple<_Elements...>>::type
>::type
get(tuple<_Elements...>& __t) noexcept
{ return __get_helper<__i>(__t); }
Note that this performs ADL to find __get_helper, which requires the associated
classes of __t to be complete, and those include B<A>. So this reduces to:
struct A;
template<typename T> struct B { T t; };
std::tuple<B<A>*> t(nullptr);
auto *p = std::get<0>(t);
... which is rejected in the same way.
