[SYCL] Return support for compilation with C++14 back

[romanovvlad]

The patch enables ability to compile a SYCL application that doesn't use certain
features (sycl::span, SYCL2020 specialization constants) with -std=c++14 option
by guarding code which uses C++17 feature with macro.

[romanovvlad]

Probably it would be nice to add a warning saying that some features are disabled in case of compilation with c++14

[s-kanaev]

LGTM

[bader]

Probably it would be nice to add a warning saying that some features are disabled in case of compilation with c++14

Please, add a warning and fix pre-commit failures.

[romanovvlad]

Probably it would be nice to add a warning saying that some features are disabled in case of compilation with c++14

Hm, not sure how to add a warning message in headers. Pragma warning is not reported as SYCL headers are considered as system ones. Any thoughts? Should we do this in the driver?

[bader]

Probably it would be nice to add a warning saying that some features are disabled in case of compilation with c++14

Hm, not sure how to add a warning message in headers. Pragma warning is not reported as SYCL headers are considered as system ones. Any thoughts? Should we do this in the driver?

+@AaronBallman
Maybe there is a way to emit a non-warning message from system headers?

[AaronBallman]

Probably it would be nice to add a warning saying that some features are disabled in case of compilation with c++14

Hm, not sure how to add a warning message in headers. Pragma warning is not reported as SYCL headers are considered as system ones. Any thoughts? Should we do this in the driver?

+@AaronBallman
Maybe there is a way to emit a non-warning message from system headers?

Some compilers support #warning and will produce the message even in a system header: https://godbolt.org/z/Yj7hsxjsv

[romanovvlad]

Probably it would be nice to add a warning saying that some features are disabled in case of compilation with c++14

Hm, not sure how to add a warning message in headers. Pragma warning is not reported as SYCL headers are considered as system ones. Any thoughts? Should we do this in the driver?

+@AaronBallman
Maybe there is a way to emit a non-warning message from system headers?

Some compilers support #warning and will produce the message even in a system header: https://godbolt.org/z/Yj7hsxjsv

Trying to add a warning here: #4303 (comment)