... for CWL-style merging of conditional branches in native Galaxy workflows.

I have CWL conditionals working down stream - the simply attach an expression to steps of the workflow and replace all the outputs with null values if the expression does not evaluate to "true". This is the CWL way of "branching" in conditionals, the matching CWL syntax for merging branches is simply to pick non-null values from various branches to continue the workflow with. This tool would is a very natural Galaxy way of doing that.

This tool could be used to build some cool conditional logic with collections without even needing the forthcoming 'when' expressions on workflow steps. Imagine for instance, we added a "Replace Failed" (or "Replace Empty") collection operation to match "Filter Failed" (or "Filter Empty") that could replace collection elements with 'null' values or a supplied default. We could then run two different algorthims across a list (e.g. one that works well for small datasets but fails out for large ones and one that works well with large datasets) and then run "Replace Failed" with null values and then map both results over this tool. Using purely data flow constructs you've picked the right tool for each element of a list.

I haven't added this to the default tool panel yet though, might be worth waiting for either those when branching expressions or more collection operation tool support.