unconditional use of std::optional breaks portability

[jeffhammond]

I am using Clang 6.0.1 on FreeBSD. This compiler, or rather the STL it uses by default, lacks support for std::optional. The compiler warning correctly suggests I use boost::optional instead.

triSYCL can achieve greater compiler/runtime portability if it uses std::optional when available but falls back on boost::optional when not.

The relatively easy fix would need to be applied to TRISYCL_PROPERTY_CREATE in include/CL/sycl/property_list.hpp. I would paste it here except that I have not figured out how to copy and paste from my FreeBSD VM into my host environment 🤦‍♂️

[keryell]

According to https://clang.llvm.org/cxx_status.html Clang 6 supports C++17 so I agree it is probably an STL problem.
While I am compassionate about supporting old environments, triSYCL is also a research prototype to investigate modern/future features and provide feedback to Khronos and ISO C++.
The internal branch we have about Xilinx ACAP devices is even more modern. At some point we will open-source it and it won't compile with Clang 6 anyway...
But the fact you are using a FreeBSD VM means to me you are more a virtual user than a real user. ;-)
What about fixing the STL of FreeBSD? That seems like a great contribution. :-)
Good news: Intel has an implementation relying on C++11 only features I think. So you have an open-source alternative that should run on your system: https://github.com/intel/llvm/blob/sycl/sycl/doc/GetStartedWithSYCLCompiler.md

[jeffhammond]

@keryell I agree with all of this. I just thought it was something easy to fix. While Clang 6 is older, I am using the latest version of FreeBSD (12.0).