After talking to Rich, it is considered an error to created a fixed buffer of size 0 and that should throw. The ticket ASYNC-143 covers that check.

On the last question, a transducer needs to be (potentially) be able to produce multiple values with something like a mapcat transducer. Without a buffer, there is no place for those buffers to go, so an unbuffered channel is insufficient.

Comment made by: alex.nixon

I vote to reopen this.

As a user this feels like a very subtle bug - the docstring says "if a transducer is supplied, a buffer must be specified". In your example you have specified a buffer – one of size zero – and so it's very surprising when you find out (at some later stage in your program) that your transducer was silently ignored. If a fixed buffer of size zero is not a valid thing to construct then I would expect (buffer 0) to throw an IllegalArgumentException or similar.

And philosophically I don't understand why the combination of an unbuffered channel and a transducer should be forbidden. What's the argument here?

I'm a dope and unbuffered channels (or 0-sized fixed buffer) channels do not support transducers per the chan docstring.

I suspect the problem is here:
https://github.com/clojure/core.async/blob/master/src/main/clojure/clojure/core/async/impl/channels.clj#L115-L133

where a take can't get a value from the buffer (there is none) and instead satisfies itself by matching up directly with a put. In this case, add! is never called so the transducer is not applied.

In the case of an expanding transducer, a buffer is needed at this point where none exists.