A few observations: this is changing the public API. Any concerns?

I assume it need not be thread-safe, am I right?

Technically the public APIs that changed are meant for the consumption by generated expression code. So I'm not too worried about changing them. Maybe they would end up in the DLLs of compiled modules? But I'm not too worried about it. I think we already broke some of these "public" APIs since the 3.4.1 release by adding CodeContext arguments.

I don't think thread-safety is an issue? Based on my (limited) understanding of these meta objects it's just generating an expression tree that transforms the given arguments to match the function signature. The whole thing seems overly complicated to me, but not something I really want to take on right now.