I can now see what you meant by completion not being left-to-right solvable. Thanks for taking the time to lay out a clear example.

While completion isn't be solvable in general, surly we can agree that its usefulness (when solvable) merits making it more readily available to users.

Having the shell implementation automatically deduce suggestions from the command itself (when available/possible) would go a long way towards increasing availability in my opinion.