After a few comments and my own consideration I've completely Edited the content (and title) of this post.
What are valid "output_layer" parameter options when scripting with the "saveselectedfeatures" tool?
I'm writing a script that uses the tool as follows:
processing.runandload('qgis:saveselectedfeatures', Input, 'Output.shp')
I also tried removing the shapefile extension and just putting 'Output' as the final parameter. Still errors and no resulting output.
Don't worry about "Input", it's a perfectly valid layer with selected features.
@Joseph provided the partial solution of replacing " 'Output.shp' " with " None " (no quotations).
processing.runandload('qgis:saveselectedfeatures', Input, None)
This results in successful execution with a temporary output layer called "Solution" popping up in the Layers Window.
I call this a partial solution because I find it hard to believe that the designer(s) of this tool didn't also intend to give users the option to choose the name of their output file (as I originally tried to do) via the final parameter ('output_layer'), as described in the tool's Help Tab.
The tool's Help tab offers the following instruction:
But it seems this is not quite as "helpful" as it could be. It seems like a few short and simple examples of acceptable values for the 2nd parameter would avoid any confusion.
Output file options: temporary layer, Shapefile, etc... 2nd Parameter ('output_layer') options: " 'User_chosen_name' " (or " 'User_chosen_name.shp' "), OR "None" (produces auto-named temporary layer).
I'm a relatively new user of QGIS.