thread safety of reflection API #6240
Description
It's sort of implemented, but was never tested. Also trees aren't synchronized at all, because it was only recently when they've been converted to abstract types.
NOTE that this issue mostly affects runtime reflection, not reflection used in macros. Compiler itself has never been thread-safe, so noone uses it from multiple threads, therefore issues might only arise if a macro spawns multiple threads, which do certain operations simultaneously.