The specification of Thread::setDaemon states:

  * <p> This method must be invoked before the thread is started.

but then has:

    * @throws IllegalThreadStateException
    * if this thread is {@linkplain #isAlive alive}

and is implemented with:

 public final void setDaemon(boolean on) {
        checkAccess();
        if (isAlive()) {
            throw new IllegalThreadStateException();
        }
        daemon = on;
    }

The problem is that not isAlive() is not the same as "before the thread has started" as a terminated thread is not alive. Hence the code allows the daemon state of a terminated thread to change - contrary to (at least part of) the specification.

This issue was discovered while investigating an apparent bug in the VM tracking the number of daemon threads - see JDK-8218483.

Given we can fix JDK-8218483 in the VM, and that this is a long standing "bug" it may not be worth fixing.