How do I use the new C++17 execution policies? [duplicate]

c++17 was not yet finalized. And various compilers have not yet fully implemented it.

-std=c++17 means "give me all of C++17 you have finished", not "be a perfectly valid C++17 compiler".

This feature is not supported by your compiler and/or standard library at this point. Check back in a few weeks/months/years.

There is no generally accepted "please give me C++17 if you fully support it, and otherwise give me an error" flag you can pass to a compiler; partly because it is of little practical use. If the subset of C++17 they provide is sufficient, then you win. And if you need a fully compliant compiler, specific versions of compilers don't know if they have bugs, so you couldn't trust the flag anyhow and would have to test it against compiler versions. And if you already know what versions of the compiler have sufficiently valid C++17, you don't need a flag to tell you.

As far as I understand from cppreferencethis feature is defined in document P0024R2 and it is not yet supported in any compiler.

If you're using g++ then you can try the non-standard extension:

https://gcc.gnu.org/onlinedocs/libstdc++/manual/parallel_mode.html