clang 4 build error on with c++1z

Question

I just updated my arch linux system to the latest which includes gcc 7.1.1. Trying to build this:

#include <functional>

int main(int argc, char** argv) {
    return 1;
}

using the command

clang++ main.cpp -std=c++1z

results in the error:

In file included from main.cpp:1:
In file included from /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/functional:60:
In file included from /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/unordered_map:47:
In file included from /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/bits/hashtable.h:37:
In file included from /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/bits/node_handle.h:39:
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/optional:1032:27: error: use of class template 'optional'
      requires template arguments
  template <typename _Tp> optional(_Tp) -> optional<_Tp>;
                          ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/optional:451:11: note: template is declared here
    class optional
          ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/optional:1032:40: error: expected ';' at end of declaration
  template <typename _Tp> optional(_Tp) -> optional<_Tp>;
                                       ^
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.1.1/../../../../include/c++/7.1.1/optional:1032:41: error: cannot use arrow operator on a type
  template <typename _Tp> optional(_Tp) -> optional<_Tp>;

Is this an error on my part, arch linux, or clang?

Update: (forgot to add versions)

gcc package version 7.1.1-2 (provides /usr/include/c++ dir)
clang package version 4.0.0-3

Ah, I didn't realize that's what was going on here (and forgot about that as a feature of c++17). Thanks! — ryan0270, May 30 '17 at 13:35
Same error here. Did you find any workaround ? The only one I found was to switch to g++.. — Matthieu G, Jun 05 '17 at 11:23
No workaround for clang. As Barry said; it's just not implemented yet. In my case it was only a buildbot issue since most of my work uses g++ anyhow but the clang test builds are still failing. — ryan0270, Jun 05 '17 at 12:42

score 7 · Answer 1 · answered Jun 28 '17 at 12:42

Little late but I would like to sum it up.

This:

template <typename _Tp> optional(_Tp) -> optional<_Tp>;

is the example of deduction guides. Here you can find nice explanation of this language feature.

New version of libstdc++ uses it in its implementation when -std=c++17 or c++1z flag is in use. Unfortunately Clang does not support this feature yet. Clang C++1z status page reports it as available only on SVN.

You can find bug report for Arch Linux here.

clang 4 build error on with c++1z

1 Answers1

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