It's going to be rough to actually rollout the latest spdlog, because it requires going through every printable type (incl. those from simbody) and annotating the relevant datastructures to say "ostream this".

OpenSim Creator's been getting this warning for a couple of years now, because it's compiling with /W4, the way to ignore it is by disabling the warning:

• https://github.com/ComputationalBiomechanicsLab/opensim-creator/blob/main/src/OpenSimCreator/CMakeLists.txt#L450

Not exactly a great solution but if you want CI back up-and-running then I'd maybe consider first disabling those warnings and then separately try to sort out the spdlog upgrade, so that you aren't getting pressure from PRs etc. to fix CI

Thanks @adamkewley, I think patching the issue by disabling warnings is better for now. I'll switch this PR to do that instead.

Seems to do the trick! - sucks to ignore warnings but it gets the project's wheels turning while evaluating what to do (imo, I'd de-integrate spdlog+fmt around the time OpenSim upgrades to C++20 on target platforms that include std::format - drops a dependency but means you can still use the "{}" syntax