[klibertp]

> For Groovy this just isn't possible

It's true in general, but in most specific cases it is possible to statically analyse dynamic languages to the point of providing meaningful auto-completion. For example in Python-land there is Jedi (http://jedi.jedidjah.ch/en/latest/), and also commercial Komodo IDE and PyCharm. This is done through static analysis, so that no code is ever run for auto-completion.

So, while it's impossible in general case, it's perfectly possible to create useful auto-completion for dynamic languages. There are such tools for Python, JS and probably others as well.