Parallel Runner Example Fails to Build on GCC 9

[jwillikers]

Expected Behavior

Parallel Runner example should build without error.

Actual Behavior

The parallel_runner fails to build with GCC 9.1.1 (devtoolset-9) on RHEL 7 and with Mingw64 (GCC 9.2) on Windows 10.
The <execution> header pulls in the exact same missing include on both platforms:

/opt/rh/devtoolset-9/root/usr/include/c++/9/pstl/parallel_backend_tbb.h:19:10: fatal error: tbb/blocked_range.h: No such file or directory
  19 | #include <tbb/blocked_range.h>
     |        ^~~~~~~~~~~~~~~~~~~~~

Steps to Reproduce the Problem

Build the parallel_runner test on RHEL 7 using devtoolset-9 with C++20 enabled.
Or
Build the parallel_runner test on Mingw64 (GCC 9.2) with C++20 enabled.

Specifications

• Version: 1.1.6
• Platform: RHEL 7 / Windows 10 (MSYS2)
• Toolchain: devtoolset-9 / Mingw64

[krzysztof-jusiak]

https://github.com/intel/tbb.git is required for gcc to use C++ parallel algorithms, it seems that gcc is not installed properly 🤔
Any parallel algorithm won't compile on such environment.
Not sure Boost.UT can fix that 🤔

@jwillikers feel free to reopen if you think that Boost.UT could handle it 👍