This would also be fixable by using VERSION_LESS for the comparison, right? Wondering if that way might be more compatible.

Oh shoot no, that won't work either because you still need to compare against the other style of version number on the right hand side. Ugh, it looks like this came in as a consequence of CMP0093 in CMake 3.15:

https://cmake.org/cmake/help/v3.15/policy/CMP0093.html

If noetic-devel is supposed to support Debian Buster and Ubuntu Focal (per ros-infrastructure/rep#202), then we're dealing with CMake 3.13.4 and 3.16.3. TBH I think the easiest fix for this might be to just disable CMP0093.

A little bit of fun further discovery— CMP0093 only applies to the FindBoost shipped by CMake, which on Focal is in /usr/share/cmake-3.16/Modules/FindBoost.cmake. However, in Boost 1.71, Boost itself also ships its own module, in /usr/lib/x86_64-linux-gnu/cmake/Boost-1.71.0/BoostConfig.cmake.

The native Boost version is first in the search path, so setting CMP0093 in either direction does nothing— it doesn't exist in CMake 3.13, and by 3.16, Boost's own module aliases the place where it would have had an effect.

My suggestion would be to do something like this:

# Compatibility with Boost < 3.15.
if (NOT Boost_VERSION_STRING)
  set(Boost_VERSION_STRING "${Boost_MAJOR_VERSION}.${Boost_MINOR_VERSION}.${Boost_SUBMINOR_VERSION}")
endif()

And then do comparisons against Boost_VERSION_STRING using VERSION_LESS and friends.

Please see #1903 for an alternative approach since the conditional logic isn't necessary anymore.

Closing in favor of #1903.