required c++ language standard

In pkgsrc, GNU Radio is at 3.10.12.0.

It is marked as needing a C++20 compiler, but this seems surprising.
This minute, I believe that's confused on our part, and it needs C++17.

Reading CMakeFile.txt and README and following pointers, I can't find
anything which says what C++ language variant GNU Radio is written in,
and whether the build environment needs to force that, vs letting the gr
build system test if --std=c++NN works and then adding it.

I find experimentally that building gr-osmosdr with a forced --std=c++11
fails due to things in the gnuradio headers, apparently, and with a
forced --std=c++17 it builds. I find the same thing with our
gnuradio-core package which is the normal key blocks split from
everything.

My questions:

Is this documented and I'm just not looking in the right place?

What C++ language variant is GNU Radio 3.10.12 written in?

Does the build system check if the compiler can support the required
version and add --std somehow? If not, do people think that's a bug?

Are things ok only because everybody but me is using a system where
the compiler is always really recent, and defaults to high language
standards?

Thanks
Greg