[llvm-bugs] [Bug 48507] New: Type traits error in c++17 version

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48507

            Bug ID: 48507
           Summary: Type traits error in c++17 version
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: krylov.yaroslav at gmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

Here is the two strings code using type traits:
```
#include "nlohmann/json.hpp"
static_assert( std::is_copy_assignable<nlohmann::ordered_json>::value, "" );
```
https://godbolt.org/z/4eqjas

It works perfectly with libstdc++, and it works with libc++ using c++11 or
c++14.
But libc++ with c++17 can't compile this code. I tried to dig into problem, to
find the difference between c++17 and non-c++17 in libc++ headers, I tried to
dig into nlohmann json to extract minimal example, but failed.

Root issue on Github: https://github.com/nlohmann/json/issues/2491

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