In the current spec, a new thread's thread local is initialized by the obj argument given to make-thread-local. There may be cases that the child thread wants to see the same value as the parent thread at the moment make-thread is called.
Java has InheritableThreadLocal that does it: https://docs.oracle.com/javase/7/docs/api/java/lang/InheritableThreadLocal.html There seems a proposal for C++ as well: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2633r0.html
