Looks like I spoke too soon. Upgraded the system to Debian testing with g++ 10.2.1 and still getting the same error. Tried building on macOS, same result:
builder@microair ~ % g++ --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 12.0.5 (clang-1205.0.22.11)
Target: x86_64-apple-darwin20.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
...
...
/Users/builder/src/arangodb/3rdParty/s2geometry/dfefe0c/src/s2/util/math/mathutil.h:168:24: error: explicit specialization of 'Round<int, double>' after instantiation
inline int32 MathUtil::Round<int32, double>(double x) { ^
...
...
Any ideas?