I tried to sum this much ask I could in a gist.
This file is the example sbt session in question.
The basic idea is that if you define a function binding for an InputKey within a config and then define an alias to that keys binding in the global scope so users won't have to type the config name :, you'll get an unexpected parse error when not using the namespace in sbt's prompt (global scope)
Parsed result dummy task not found in ...
I am guessing the dummy task is provided by the inputTask method and is missing some scope registration.
I originally thought this had to do with the custom parser combinators but I am also about to reproduce this with the default parser.
IMPORTANT This project has moved to https://github.com/sbt/sbt. This issue still exists to avoid dead links, but GitHub limitations prevent the original issue from being correctly preserved here. Please see sbt/sbt#114 for the full issue and to comment.