[Bug c++/105386] New: Tuple in unevaluated context is instantiated; creates reference to void

[Bug c++/105386] New: Tuple in unevaluated context is instantiated; creates reference to void

[cuzdav at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105386

            Bug ID: 105386
           Summary: Tuple in unevaluated context is instantiated; creates
                    reference to void
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cuzdav at gmail dot com
  Target Milestone: ---

Created attachment 52878
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52878&action=edit
preprocessed code

Starting with g++11.2 some of meta-programming code started to fail.
While instantiating a tuple in an unevaluated context, it now complains that
the tuple contains a void member and it's creating a reference to void, yet it
is the same type as in the working case, just computed differently.


Output of g++ -v
(base) bash-4.2$ /opt/imc/gcc-11.2.0/bin/g++ -v
Using built-in specs.
COLLECT_GCC=/opt/imc/gcc-11.2.0/bin/g++
COLLECT_LTO_WRAPPER=/opt/imc/gcc-11.2.0/libexec/gcc/x86_64-pc-linux-gnu/11.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-11.2.0/configure --prefix=/opt/imc/gcc-11.2.0
--enable-languages=c,c++,fortran,lto --disable-multilib
--with-build-time-tools=/opt/buildagent/work/d7a993299378ca7e/11.2.0/INSTALLDIR//opt/imc/gcc-11.2.0/bin
--enable-libstdcxx-time=rt
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.2.0 (GCC) 



Code appears in its entirety in attachment, but here is the main focus of the
error:

class Chunk { };
using ChunkMetaData = std::tuple<void, Chunk, void>;

template <typename MessageT> ChunkMetaData chunkMetaData(MessageT&&);


template <typename MessageChunkT>
class ChunkSeq
{
public:

  ////////////////////////////////////////
  using good_type = ChunkMetaData;


#ifdef SHOWBUG
  using bad_type = decltype(chunkMetaData(std::declval<MessageChunkT>()));
  static_assert(std::is_same_v<good_type, bad_type>);
  using MetaData = bad_type;
#else
  using MetaData = good_type;
#endif

  ////////////////////////////////////////


In gcc10, when SHOWBUG is defined, it still builds.  The static assertion
passes.  With gcc11 and 12, it fails, upset about a tuple creating a reference
to void.


Live example:
https://godbolt.org/z/4Wq3s936W


Errors:
In file included from krx/feed/sr10/foo.cpp:1:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:
In instantiation of ‘struct std::_Head_base<2, void, false>’:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:407:12:
  required from ‘struct std::_Tuple_impl<2, void>’
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:244:12:
  required from ‘struct std::_Tuple_impl<0, void, Chunk, void>’
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:599:11:
  required from ‘class std::tuple<void, Chunk, void>’
krx/feed/sr10/foo.cpp:171:42:   [ skipping 2 instantiation contexts, use
-ftemplate-backtrace-limit=0 to disable ]
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<FieldIsPartialBegin<Chunk> >::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<FieldIsPartialBegin<Chunk> >,
std::tuple<PartialField<0>, PartialField<1> > >’
krx/feed/sr10/foo.cpp:235:7:   required by substitution of ‘template<class T>
using PartialStartField = typename KeepIf<FieldIsPartialBegin<T>, typename
RemoveIfImpl<InvertPredicate<ChunkHasPartialType<T> >,
std::tuple<PartialField<0>, PartialField<1> > >::type>::type [with T = Chunk]’
krx/feed/sr10/foo.cpp:240:47:   required from ‘constexpr const bool
HasPartialBegin::value<Chunk>’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<HasPartialBegin>::value<Chunk>’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<HasPartialBegin>, std::tuple<Chunk> >’
krx/feed/sr10/foo.cpp:247:6:   required from here
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:182:17:
error: forming reference to void
  182 |       constexpr _Head_base(const _Head& __h)
      |                 ^~~~~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:219:7:
error: forming reference to void
  219 |       _M_head(_Head_base& __b) noexcept { return __b._M_head_impl; }
      |       ^~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:222:7:
error: forming reference to void
  222 |       _M_head(const _Head_base& __b) noexcept { return
__b._M_head_impl; }
      |       ^~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:224:13:
error: ‘std::_Head_base<_Idx, _Head, false>::_M_head_impl’ has incomplete type
  224 |       _Head _M_head_impl;
      |             ^~~~~~~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:224:13:
error: invalid use of ‘void’
In file included from krx/feed/sr10/foo.cpp:1:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:
In instantiation of ‘struct std::_Tuple_impl<2, void>’:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:244:12:
  recursively required from ‘struct std::_Tuple_impl<1, Chunk, void>’
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:244:12:
  required from ‘struct std::_Tuple_impl<0, void, Chunk, void>’
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:599:11:
  required from ‘class std::tuple<void, Chunk, void>’
krx/feed/sr10/foo.cpp:171:42:   required from ‘class ChunkSeq<Chunk>’
krx/feed/sr10/foo.cpp:231:35:   required from ‘constexpr const bool
FieldIsPartialBegin<Chunk>::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<FieldIsPartialBegin<Chunk> >::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<FieldIsPartialBegin<Chunk> >,
std::tuple<PartialField<0>, PartialField<1> > >’
krx/feed/sr10/foo.cpp:235:7:   required by substitution of ‘template<class T>
using PartialStartField = typename KeepIf<FieldIsPartialBegin<T>, typename
RemoveIfImpl<InvertPredicate<ChunkHasPartialType<T> >,
std::tuple<PartialField<0>, PartialField<1> > >::type>::type [with T = Chunk]’
krx/feed/sr10/foo.cpp:240:47:   required from ‘constexpr const bool
HasPartialBegin::value<Chunk>’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<HasPartialBegin>::value<Chunk>’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<HasPartialBegin>, std::tuple<Chunk> >’
krx/feed/sr10/foo.cpp:247:6:   required from here
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:415:7:
error: forming reference to void
  415 |       _M_head(_Tuple_impl& __t) noexcept { return _Base::_M_head(__t);
}
      |       ^~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:418:7:
error: forming reference to void
  418 |       _M_head(const _Tuple_impl& __t) noexcept { return
_Base::_M_head(__t); }
      |       ^~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:425:7:
error: forming reference to void
  425 |       _Tuple_impl(const _Head& __head)
      |       ^~~~~~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:466:9:
error: forming reference to void
  466 |         _Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
      |         ^~~~~~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:
In instantiation of ‘struct std::_Tuple_impl<1, Chunk, void>’:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:244:12:
  required from ‘struct std::_Tuple_impl<0, void, Chunk, void>’
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:599:11:
  required from ‘class std::tuple<void, Chunk, void>’
krx/feed/sr10/foo.cpp:171:42:   required from ‘class ChunkSeq<Chunk>’
krx/feed/sr10/foo.cpp:231:35:   required from ‘constexpr const bool
FieldIsPartialBegin<Chunk>::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<FieldIsPartialBegin<Chunk> >::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<FieldIsPartialBegin<Chunk> >,
std::tuple<PartialField<0>, PartialField<1> > >’
krx/feed/sr10/foo.cpp:235:7:   required by substitution of ‘template<class T>
using PartialStartField = typename KeepIf<FieldIsPartialBegin<T>, typename
RemoveIfImpl<InvertPredicate<ChunkHasPartialType<T> >,
std::tuple<PartialField<0>, PartialField<1> > >::type>::type [with T = Chunk]’
krx/feed/sr10/foo.cpp:240:47:   required from ‘constexpr const bool
HasPartialBegin::value<Chunk>’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<HasPartialBegin>::value<Chunk>’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<HasPartialBegin>, std::tuple<Chunk> >’
krx/feed/sr10/foo.cpp:247:6:   required from here
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:269:7:
error: forming reference to void
  269 |       _Tuple_impl(const _Head& __head, const _Tail&... __tail)
      |       ^~~~~~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:319:9:
error: forming reference to void
  319 |         _Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
      |         ^~~~~~~~~~~
In file included from krx/feed/sr10/foo.cpp:1:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:
In instantiation of ‘struct std::_Head_base<0, void, false>’:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:244:12:
  required from ‘struct std::_Tuple_impl<0, void, Chunk, void>’
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:599:11:
  required from ‘class std::tuple<void, Chunk, void>’
krx/feed/sr10/foo.cpp:171:42:   required from ‘class ChunkSeq<Chunk>’
krx/feed/sr10/foo.cpp:231:35:   required from ‘constexpr const bool
FieldIsPartialBegin<Chunk>::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<FieldIsPartialBegin<Chunk> >::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<FieldIsPartialBegin<Chunk> >,
std::tuple<PartialField<0>, PartialField<1> > >’
krx/feed/sr10/foo.cpp:235:7:   required by substitution of ‘template<class T>
using PartialStartField = typename KeepIf<FieldIsPartialBegin<T>, typename
RemoveIfImpl<InvertPredicate<ChunkHasPartialType<T> >,
std::tuple<PartialField<0>, PartialField<1> > >::type>::type [with T = Chunk]’
krx/feed/sr10/foo.cpp:240:47:   required from ‘constexpr const bool
HasPartialBegin::value<Chunk>’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<HasPartialBegin>::value<Chunk>’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<HasPartialBegin>, std::tuple<Chunk> >’
krx/feed/sr10/foo.cpp:247:6:   required from here
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:182:17:
error: forming reference to void
  182 |       constexpr _Head_base(const _Head& __h)
      |                 ^~~~~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:219:7:
error: forming reference to void
  219 |       _M_head(_Head_base& __b) noexcept { return __b._M_head_impl; }
      |       ^~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:222:7:
error: forming reference to void
  222 |       _M_head(const _Head_base& __b) noexcept { return
__b._M_head_impl; }
      |       ^~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:224:13:
error: ‘std::_Head_base<_Idx, _Head, false>::_M_head_impl’ has incomplete type
  224 |       _Head _M_head_impl;
      |             ^~~~~~~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:224:13:
error: invalid use of ‘void’
In file included from krx/feed/sr10/foo.cpp:1:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:
In instantiation of ‘struct std::_Tuple_impl<0, void, Chunk, void>’:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:599:11:
  required from ‘class std::tuple<void, Chunk, void>’
krx/feed/sr10/foo.cpp:171:42:   required from ‘class ChunkSeq<Chunk>’
krx/feed/sr10/foo.cpp:231:35:   required from ‘constexpr const bool
FieldIsPartialBegin<Chunk>::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<FieldIsPartialBegin<Chunk> >::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<FieldIsPartialBegin<Chunk> >,
std::tuple<PartialField<0>, PartialField<1> > >’
krx/feed/sr10/foo.cpp:235:7:   required by substitution of ‘template<class T>
using PartialStartField = typename KeepIf<FieldIsPartialBegin<T>, typename
RemoveIfImpl<InvertPredicate<ChunkHasPartialType<T> >,
std::tuple<PartialField<0>, PartialField<1> > >::type>::type [with T = Chunk]’
krx/feed/sr10/foo.cpp:240:47:   required from ‘constexpr const bool
HasPartialBegin::value<Chunk>’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<HasPartialBegin>::value<Chunk>’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<HasPartialBegin>, std::tuple<Chunk> >’
krx/feed/sr10/foo.cpp:247:6:   required from here
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:254:7:
error: forming reference to void
  254 |       _M_head(_Tuple_impl& __t) noexcept { return _Base::_M_head(__t);
}
      |       ^~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:257:7:
error: forming reference to void
  257 |       _M_head(const _Tuple_impl& __t) noexcept { return
_Base::_M_head(__t); }
      |       ^~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:269:7:
error: forming reference to void
  269 |       _Tuple_impl(const _Head& __head, const _Tail&... __tail)
      |       ^~~~~~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:319:9:
error: forming reference to void
  319 |         _Tuple_impl(allocator_arg_t __tag, const _Alloc& __a,
      |         ^~~~~~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:
In instantiation of ‘class std::tuple<void, Chunk, void>’:
krx/feed/sr10/foo.cpp:171:42:   required from ‘class ChunkSeq<Chunk>’
krx/feed/sr10/foo.cpp:231:35:   required from ‘constexpr const bool
FieldIsPartialBegin<Chunk>::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<FieldIsPartialBegin<Chunk> >::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<FieldIsPartialBegin<Chunk> >,
std::tuple<PartialField<0>, PartialField<1> > >’
krx/feed/sr10/foo.cpp:235:7:   required by substitution of ‘template<class T>
using PartialStartField = typename KeepIf<FieldIsPartialBegin<T>, typename
RemoveIfImpl<InvertPredicate<ChunkHasPartialType<T> >,
std::tuple<PartialField<0>, PartialField<1> > >::type>::type [with T = Chunk]’
krx/feed/sr10/foo.cpp:240:47:   required from ‘constexpr const bool
HasPartialBegin::value<Chunk>’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<HasPartialBegin>::value<Chunk>’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<HasPartialBegin>, std::tuple<Chunk> >’
krx/feed/sr10/foo.cpp:247:6:   required from here
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:621:72:
error: forming reference to void
  621 |           _TCC<_Cond>::template
__is_implicitly_constructible<_Args...>(),
      |          
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:627:72:
error: forming reference to void
  627 |           _TCC<_Cond>::template
__is_explicitly_constructible<_Args...>(),
      |          
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:621:72:
error: forming reference to void
  621 |           _TCC<_Cond>::template
__is_implicitly_constructible<_Args...>(),
      |          
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:627:72:
error: forming reference to void
  627 |           _TCC<_Cond>::template
__is_explicitly_constructible<_Args...>(),
      |          
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
In file included from krx/feed/sr10/foo.cpp:1:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:888:71:
error: forming reference to void
  888 |       operator=(typename conditional<__assignable<const
_Elements&...>(),
      |                                     
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:888:71:
error: no matching function for call to ‘std::tuple<void, Chunk,
void>::<expression error>()’
In file included from krx/feed/sr10/foo.cpp:1:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:633:9:
note: candidate: ‘template<class ... _UElements> static constexpr
std::__enable_if_t<(sizeof... (_UElements) == sizeof... (_Elements)), bool>
std::tuple<_Elements>::__assignable() [with _UElements = {_UElements ...};
_Elements = {void, Chunk, void}]’
  633 |         __assignable()
      |         ^~~~~~~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:633:9:
note:   template argument deduction/substitution failed:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:
In instantiation of ‘static constexpr std::__enable_if_t<(sizeof...
(_UElements) == sizeof... (_Elements)), bool>
std::tuple<_Elements>::__assignable() [with _UElements = {void, Chunk, void};
_Elements = {void, Chunk, void}; std::__enable_if_t<(sizeof... (_UElements) ==
sizeof... (_Elements)), bool> = bool]’:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:899:64:
  required from ‘class std::tuple<void, Chunk, void>’
krx/feed/sr10/foo.cpp:171:42:   required from ‘class ChunkSeq<Chunk>’
krx/feed/sr10/foo.cpp:231:35:   required from ‘constexpr const bool
FieldIsPartialBegin<Chunk>::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<FieldIsPartialBegin<Chunk> >::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<FieldIsPartialBegin<Chunk> >,
std::tuple<PartialField<0>, PartialField<1> > >’
krx/feed/sr10/foo.cpp:235:7:   required by substitution of ‘template<class T>
using PartialStartField = typename KeepIf<FieldIsPartialBegin<T>, typename
RemoveIfImpl<InvertPredicate<ChunkHasPartialType<T> >,
std::tuple<PartialField<0>, PartialField<1> > >::type>::type [with T = Chunk]’
krx/feed/sr10/foo.cpp:240:47:   required from ‘constexpr const bool
HasPartialBegin::value<Chunk>’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<HasPartialBegin>::value<Chunk>’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<HasPartialBegin>, std::tuple<Chunk> >’
krx/feed/sr10/foo.cpp:247:6:   required from here
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:634:68:
error: forming reference to void
  634 |         { return __and_<is_assignable<_Elements&,
_UElements>...>::value; }
      |                                                                   
^~~~~
In file included from krx/feed/sr10/foo.cpp:1:
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:
In instantiation of ‘class std::tuple<void, Chunk, void>’:
krx/feed/sr10/foo.cpp:171:42:   required from ‘class ChunkSeq<Chunk>’
krx/feed/sr10/foo.cpp:231:35:   required from ‘constexpr const bool
FieldIsPartialBegin<Chunk>::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<FieldIsPartialBegin<Chunk> >::value<PartialField<0> >’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<FieldIsPartialBegin<Chunk> >,
std::tuple<PartialField<0>, PartialField<1> > >’
krx/feed/sr10/foo.cpp:235:7:   required by substitution of ‘template<class T>
using PartialStartField = typename KeepIf<FieldIsPartialBegin<T>, typename
RemoveIfImpl<InvertPredicate<ChunkHasPartialType<T> >,
std::tuple<PartialField<0>, PartialField<1> > >::type>::type [with T = Chunk]’
krx/feed/sr10/foo.cpp:240:47:   required from ‘constexpr const bool
HasPartialBegin::value<Chunk>’
krx/feed/sr10/foo.cpp:111:57:   required from ‘constexpr const bool
InvertPredicate<HasPartialBegin>::value<Chunk>’
krx/feed/sr10/foo.cpp:122:11:   required from ‘struct
RemoveIfImpl<InvertPredicate<HasPartialBegin>, std::tuple<Chunk> >’
krx/feed/sr10/foo.cpp:247:6:   required from here
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:899:64:
  in ‘constexpr’ expansion of ‘std::tuple<void, Chunk,
void>::__assignable<void, Chunk, void>()’
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:899:7:
error: ‘constexpr’ call flows off the end of the function
  899 |       operator=(typename conditional<__assignable<_Elements...>(),
      |       ^~~~~~~~
/opt/imc/gcc-11.2.0/lib/gcc/x86_64-pc-linux-gnu/11.2.0/../../../../include/c++/11.2.0/tuple:899:64:
note: in template argument for type ‘bool’
  899 |       operator=(typename conditional<__assignable<_Elements...>(),
      |                                      ~~~~~~~~~~~~~~~~~~~~~~~~~~^~

[Bug c++/105386] [11/12 Regression] Tuple in unevaluated context is instantiated; creates reference to void

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105386

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Tuple in unevaluated        |[11/12 Regression] Tuple in
                   |context is instantiated;    |unevaluated context is
                   |creates reference to void   |instantiated; creates
                   |                            |reference to void
           Keywords|                            |needs-bisection,
                   |                            |rejects-valid
   Last reconfirmed|                            |2022-04-26
             Status|UNCONFIRMED                 |NEW
      Known to fail|                            |11.2.1, 12.0
   Target Milestone|---                         |11.4
      Known to work|                            |10.3.1
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  clang 11 also accepts it.

[Bug c++/105386] [11/12 Regression] Tuple in unevaluated context is instantiated; creates reference to void

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105386

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|needs-bisection             |
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Started with r11-434-g115232b778943be075fc4df991e03d9387563114

[Bug c++/105386] [11/12 Regression] Tuple in unevaluated context is instantiated; creates reference to void

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105386

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org

[Bug c++/105386] [11/12 Regression] Tuple in unevaluated context is instantiated; creates reference to void

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105386

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:b6a48401da51e9042b6f0822d532b3b472492658

commit r12-8270-gb6a48401da51e9042b6f0822d532b3b472492658
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Apr 26 10:53:38 2022 -0400

    c++: decltype of non-dependent call of class type [PR105386]

    We need to pass tf_decltype when instantiating a non-dependent decltype
    operand, like tsubst does in the dependent case, so that we don't force
    completion of a prvalue operand's class type.

            PR c++/105386

    gcc/cp/ChangeLog:

            * semantics.cc (finish_decltype_type): Pass tf_decltype to
            instantiate_non_dependent_expr_sfinae.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/decltype81.C: New test.

[Bug c++/105386] [11 Regression] Tuple in unevaluated context is instantiated; creates reference to void

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105386

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[11/12 Regression] Tuple in |[11 Regression] Tuple in
                   |unevaluated context is      |unevaluated context is
                   |instantiated; creates       |instantiated; creates
                   |reference to void           |reference to void

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 12 so far.

[Bug c++/105386] [11 Regression] Tuple in unevaluated context is instantiated; creates reference to void

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105386

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:8969d00bf16a8f4de79306010fca8f14f91c171d

commit r11-9947-g8969d00bf16a8f4de79306010fca8f14f91c171d
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Apr 26 10:53:38 2022 -0400

    c++: decltype of non-dependent call of class type [PR105386]

    We need to pass tf_decltype when instantiating a non-dependent decltype
    operand, like tsubst does in the dependent case, so that we don't force
    completion of a prvalue operand's class type.

            PR c++/105386

    gcc/cp/ChangeLog:

            * semantics.c (finish_decltype_type): Pass tf_decltype to
            instantiate_non_dependent_expr_sfinae.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/decltype81.C: New test.

    (cherry picked from commit b6a48401da51e9042b6f0822d532b3b472492658)

[Bug c++/105386] [11 Regression] Tuple in unevaluated context is instantiated; creates reference to void

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105386

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 11.4/12, thanks for the bug report.