Non-conformant testing of template-template-args c++ standard feature

[mizvekov]

This project includes workarounds for the changes introduced in https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0522r0.html, which was adopted as a defect report and thus retroactively applies back to all language standards.

See here:

xtensor/include/xtensor/xutils.hpp

Line 1022 in d9c3782

#if defined(__GNUC__) && __GNUC__ > 6 && !defined(__clang__) && __cplusplus >= 201703L

The problem is that this workaround is testing for GCC specifically and C++17, excluding Clang.

GCC has included this change for a while, but it's only enabled by default in C++17 and above, but can be enabled and disabled freely with a flag as well.

Clang has never enabled it by default until now, deferring it to the flag only. This is changing with the next version, where it's going to be enabled in all language versions, and the flag becomes deprecated, to be removed in the version after that.

This means building this project is failing when this feature's enablement differs from default. With the next version of Clang, it's not going to work with the default there either.

Solution: replace above check with testing of standard feature macro: https://en.cppreference.com/w/cpp/feature_test#cpp_template_template_args

References:
llvm/llvm-project#89807
llvm/llvm-project#91504

[mizvekov]

There is another place that looks like a candidate for the same fix:

xtensor/include/xtensor/xexpression_traits.hpp

Line 106 in d9c3782

#if defined(__GNUC__) && (__GNUC__ > 6)

A user reported here that this also fixes a similar issue: llvm/llvm-project#89807 (comment)