Add C++17 support

[kazk]

Need Clang 5.

[oschonrock]

+1

C++14 is 5 yrs old and we are nearing c++20. It's frustrating to have to code in "legacy" style

What version of clang are you running that C++17 is not an option?

[kazk]

What version of clang are you running that C++17 is not an option?

C++ still uses super old Clang 3.8 :(

There's also some problems with using abandoned test framework Igloo. I'm not sure how well it'll work with C++17 (#658).
I'd like to eventually use Catch2 instead, but will need to support both until existing kata are migrated (or let C++17 kata use Catch2 and older ones use Igloo).

I haven't spent much time on this yet since we haven't had much demand.

We already have Clang 8 for C, so I'll start experimenting with updating C++ soon.

[hobovsky]

-O2 please?

[kazk]

Deployed C++17 support. Forgot to add -O2, but I can add it later. It'll probably make the compile time slightly longer, but it shouldn't be too bad. Will do some testing.

Please help with updating 57 incompatible kata. See List of C++ Kata to Update to C++17.

[hobovsky]

I tried to update two or three kata from "needs update" list (for example, "https://www.codewars.com/kata/multiplying-numbers-as-strings") and it turns out that they suffer from timeouts, because execution times for old C++14 seem to be ~60% of execution time after switching to C++17. I am not sure which part takes longer, compilation, container setup or tests themselves, but it appears that C++17 is almost two times slower.

And just to give some insight why it would be nice to have -O2: currently this kata has upper bound of 50 for C++14 and 1500 for JavaScript. Will it have to be dumbed down to 30 for C++17?

[kazk]

Maybe that's because C++17 one is using libc++ which is not the default on Linux. It was just easier for me to install the correct version from https://apt.llvm.org/ along with Clang.

libstdc++ is the default on Linux and C++14 uses it. I'll look into which version of libstdc++ is available and see how the performance is.

Never mind, C++14 one also has -stdlib=libc++.

[kazk]

I got Clang 8 with libstdc++-9 working locally. There's many more incompatible kata now, but it's much faster I think. At least "Multiplying numbers as strings" doesn't timeout.

I'll have to re-run the script to validate C++17 compatibility and update the list after deploying this.