warning STL4010: Various members of std::allocator are deprecated in C++17

[risa2000]

I am leaving it here for the record as I have been facing it in both VS 2017 and VS 2019 Community versions recently. When building a simple test code:

//#define _SILENCE_CXX17_OLD_ALLOCATOR_MEMBERS_DEPRECATION_WARNING

#include <xtensor/xarray.hpp>

int main(int argc, char* argv[])
{
    xt::xarray<double> test = { 1.0, 0.5, 0.1, 0.001, 0.0001 };
    return 0;
}

with /std:c++17 the compiler produces a heap of warnings of the type:

warning STL4010: Various members of std::allocator are deprecated in C++17

This is related to the deprecated allocator implementation in C++17 (as detailed here https://devblogs.microsoft.com/cppblog/c17-feature-removals-and-deprecations/).

The warnings can be masked by the #define, but it seems to be only temporary solution.

Incidentally, I have noticed that some other projects already addressed this problem (e.g. nlohmann/json#872) and the fix seems to be rather light, but I am not that familiar with the STL to propose the fix myself.

The test code build log for the reference (from ninja) is here:
xtest.build.log

[JohanMabille]

Thanks for reporting this issue!

Indeed the fix seems quite simple, some calls must be replaced with indirect calls through allcator_traits