error: no template named 'is_same_v' in namespace 'std'; did you mean 'is_same'?

Question

This is a followup for another question which is already resolved. But then, I receive this error:

error: no template named 'is_same_v' in namespace 'std'; did you mean 'is_same'?
    !std::is_same_v<typename std::iterator_traits<T>::value_type, void>, O
     ~~~~~^~~~~~~~~
          is_same
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/type_traits:831:61: note: 'is_same' declared here
template <class _Tp, class _Up> struct _LIBCPP_TEMPLATE_VIS is_same           : public false_type {};
                                                            ^

Based on previous answer :

... it was added in C++14, as were most of the other type trait *_t versions... C++17 added *_v versions as inline constexpr variables...

So, is_same_v is added by C++17. But I already add C++14 and C++17 to QMake by:

CONFIG += c++14
CONFIG += c++17
QMAKE_CXXFLAGS += -std=c++14
QMAKE_CXXFLAGS += -std=c++17

Is it possible that XCode (Clang) lacks some C++17 features? My Xcode version is 9.4.1 and my Clang++ version is:

$ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -v
Apple LLVM version 9.1.0 (clang-902.0.39.2)
Target: x86_64-apple-darwin17.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

Code

There are both C++14 *_t type trait versions and C++17 *_v versions in the code which is from a repository with many contributors:

template<class T, class O = T>
using IteratorOnly = std::enable_if_t<
    !std::is_same_v<typename std::iterator_traits<T>::value_type, void>, O
>;

Observation

When I use CONFIG += c++14 in QMake project file, I receive only this error:

• enable_if_t is fine and is_same_v is bad

error: no template named 'is_same_v' in namespace 'std'; did you mean 'is_same'?

When I use CONFIG += c++17 in QMake project file, I receive both these error:

• both enable_if_t and is_same_v are bad

error: no template named 'enable_if_t' in namespace 'std'; did you mean 'enable_if'?

error: no template named 'is_same_v' in namespace 'std'; did you mean 'is_same'?

Why?

Answer 1

The tool chain was not capable of C++17. Therefore, eventually, replaced std::is_same_v with std::is_same with this approach ...