> I wouldn't be OK with magic switching in the behaviour of ContextDecorator (that's not only semantically confusing, it's also going to make the contextlib function wrappers even slower than they already are).

I hear you on the semantic confusion, but is a single check at definition time really that expensive? The runtime cost is zero.

> I'm also entirely unclear on what you would expect a synchronous context manager to do when applied to an asynchronous function, as embedding an "await" call inside a synchronous with statement is unlikely to end well.

It would be like:

async def blah():
    with something():
        await foo()

There's nothing weird about this; people write the long version all the time. You'd only do it when 'something()' doesn't do I/O, but there are lots of context managers that don't do I/O.