Bug#877838: clang-5.0: c++17 std::get(std::variant) fails to compile with libstdc++

[Daniele Di Proietto]

Package: clang-5.0
Version: 1:5.0~+rc2-1
Severity: normal

Dear Maintainer,

I'm trying to compile c++17 code that uses `std::get(std::variant)`.
It appears to work with g++-7, but it fails with clang++-5.0.

$ cat variant.cc 
#include <variant>

int main() {
  std::variant<int, float> v{42};
  int i = std::get<int>(v);
}

$ g++-7 -std=c++1z variant.cc -o variant
$ clang++-5.0 -std=c++1z variant.cc -o variant
In file included from variant.cc:1:
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/variant:235:46: error: cannot cast 'std::variant<int, float>' to its
      private base class 'std::__detail::__variant::_Variant_storage<true, int, float>'
      return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
                                             ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/variant:1192:35: note: in instantiation of function template specialization
      'std::__detail::__variant::__get<0, std::variant<int, float> &>' requested here
      return __detail::__variant::__get<_Np>(__v);
                                  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/variant:739:14: note: in instantiation of function template specialization
      'std::get<0, int, float>' requested here
      return get<__detail::__variant::__index_of_v<_Tp, _Types...>>(__v);
             ^
variant.cc:5:16: note: in instantiation of function template specialization 'std::get<int, int, float>' requested here
  int i = std::get<int>(v);
               ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/variant:878:7: note: constrained by private inheritance here
    : private __detail::__variant::_Variant_base<_Types...>,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/variant:235:74: error: '_M_u' is a private member of
      'std::__detail::__variant::_Variant_storage<true, int, float>'
      return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
                                                                         ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/variant:878:7: note: constrained by private inheritance here
    : private __detail::__variant::_Variant_base<_Types...>,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/variant:377:34: note: member is declared here
      _Variadic_union<_Types...> _M_u;
                                 ^
2 errors generated.

Unrelated: I've tried with libc++ 4.0.1-1 from unstable, but I see a failure similar to #855222

Please let me know if additional information is needed.

Thanks,

Daniele

-- System Information:
Debian Release: 9.1
  APT prefers stable
  APT policy: (900, 'stable'), (500, 'unstable-debug'), (500, 'testing-debug'), (500, 'stable-debug')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-3-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages clang-5.0 depends on:
ii  binutils                 2.28-5
ii  libc6                    2.24-14
ii  libc6-dev                2.24-14
ii  libclang-common-5.0-dev  1:5.0~+rc2-1
ii  libclang1-5.0            1:5.0~+rc2-1
ii  libgcc-6-dev             6.3.0-18
ii  libgcc1                  1:7.1.0-7
ii  libjsoncpp1              1.7.4-3
ii  libllvm5.0               1:5.0~+rc2-1
ii  libobjc-6-dev            6.3.0-18
ii  libstdc++-6-dev          6.3.0-18
ii  libstdc++6               7.1.0-7
ii  libtinfo5                6.0+20161126-1
ii  zlib1g                   1:1.2.8.dfsg-5

Versions of packages clang-5.0 recommends:
pn  llvm-5.0-dev  <none>
ii  python        2.7.13-2

Versions of packages clang-5.0 suggests:
pn  clang-5.0-doc  <none>
pn  gnustep        <none>
pn  gnustep-devel  <none>
pn  libomp-dev     <none>

-- debconf-show failed