[llvm-bugs] [Bug 47261] New: <version> header is missing __cpp_lib_type_identity and other macros

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47261

            Bug ID: 47261
           Summary: <version> header is missing __cpp_lib_type_identity
                    and other macros
           Product: libc++
           Version: 11.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: llvm at marehr.dialup.fu-berlin.de
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

Hi clang-team,

the following code doesn't compile, even thought it can compile
`std::type_identity` and `std::unwrap_reference` just fine since clang 9.0.0.

```c++
#include <type_traits>

using a = std::type_identity<int>; // works fine
using b = std::unwrap_reference_t<int>; // works fine

#ifndef __cpp_lib_type_identity
#error "Not defined?"
#endif

#ifndef __cpp_lib_unwrap_ref
#error "Not defined?"
#endif
```

https://godbolt.org/z/6vaa6x

I thought https://eel.is/c++draft/version.syn suggests that
__cpp_lib_type_identity should be defined if it was implemented.

Thank you!

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