@jleppert This works for me using the gcc-10 branch. I even used raspberrypicm4-ioboard and 2.71.3+rev5 to be sure. Can you confirm from the top of your build log that gcc-10 is being installed?
Selecting previously unselected package g++-10.
Preparing to unpack .../14-g++-10_10.2.1-6_arm64.deb ...
Unpacking g++-10 (10.2.1-6) ...
Preparing to unpack .../15-g++_4%3a10.2.1-1_arm64.deb ...
Unpacking g++ (4:10.2.1-1) over (4:8.3.0-1) ...
(Reading database ... 20908 files and directories currently installed.)
Removing g++-8 (8.3.0-6) ...
Removing gcc-8 (8.3.0-6) ...