Because it is a linker flag and clang prints a warning otherwise.

Tests for Vector<MultiPrecision> were removed by TO...

Fixed definition of static members in communicators when the header files are used in multiple translation units

In most cases it just makes the error messages longer without helping
much (you would need to use clang if you actually want more useful error
messages). And nvcc prints some useless warning because of it...

Works fine with g++ 5.5 and nvcc 8.0 in Arch Linux. File a bug report on
Gitlab if you have problems.

This is getting really cumbersome and error-prone, somebody should
figure out how to do this more generally for all possible combinations
of CV-qualifications...

The link provided in the comment is not related - tnl-cuda-arch is not
built as a static binary.

Anyway, I don't see any problem with this. Either fix it properly or at
least file a full bug report on Gitlab. Upgrading cmake on your system
might also help...