I use thread_local a lot, but until recently, on Windows a delay-loaded dll with thread_local would've not worked, and the fix that is in place today is costly,

I use thread_local a lot, but until recently, on Windows a delay-loaded dll with thread_local would've not worked, and the fix that is in place today is costly, okay that may not be the typical case, but it shows that support for such feature can create a lot of cost elsewhere. Another pitfall with these is with thread-stealing concurrent schedulers - e.g. your worker thread now waits on something, and the scheduler decides to reuse the current thread for another worker - what is the meaning of thread_local there? Another one would be coroutines (though haven't used them a lot in C/C++).