Merge lists from config file with lists provided in CLI options

[Aposhian]

Currently, I specify options to a list like:

{
    "build": {
        "mixin": [ "ccache", "rel-with-deb-info" ]
    }
}

then it works just fine, but if I add an additional option to the list via the CLI like:

colcon build --mixin asan-gcc

then the whole list is ignored from my config file, which is rather inconvenient: you must use either all of the defaults and no more, or none of the defaults.

If the lists from the config files were simply merged with options provided on the CLI, then my use case would be easier, but the only way to disable the defaults would be to use --ignore-user-meta, which will apply to all of your defaults, not just one list.

[christophebedard]

Some context on why merging options is not really straightforward can be found here: colcon/colcon-mixin#18 (comment).

It might be possible to improve this to do the merging carefully on a case-by-case (option by option) basis, see colcon/colcon-mixin#18 (comment).