On OpenBSD (and probably FreeBSD), there is a build error due to a missing namespace-terminating brace in src/mongo/platform/stack_locator_unknown.cpp.

In file included from src/mongo/platform/stack_locator_openbsd.cpp:29:0:
src/mongo/platform/stack_locator_unknown.cpp:37:30: error: expected '}' at end of input
StackLocator::StackLocator() = default;