Since theprevious thread, I often ran into troubles with constraining sketch. Very often quite some time was spent on adjusting here and there without definite direction, and suddenly the issue was gone. This made me trapped in anxiety everytime creating a sketch with splines to be constrained.
Below is an example, in which I was told "Sketch nees 0 constraints" and in fact the angle (34.2°) is free. If the angle is fixed, the sketch is onverconstrained.
I found how to fix it after trial and error ---- simply moving the driving dimension "b=30.0" to other objects, e.g. the short line in the middle. But I don't know why it works. That means, I don't know whether I can have good luck to fix similar issue next time.