Smart Object as Layer Mask?

how about vector masking? these can be easily edited. As a mask is able to be edited an unlimited amount, why the need for a smart object? create a "Quick mask" and save it in the channels by renaming it if you want a mask you can go back to if you mess up. 

PhotoLine is the only layer-based image editor that is able to use smart objects (called "placeholders" in PL) as layer masks. PL's layer stack is much more forgiving and flexible - supporting an unlimited number of any kind of layer and layer group as layer masks, even with live layer referenced clones. I wish Photoshop's aging layer stack would decouple layer masks from its layers. Clipping masks are an awkward work-around.

Node-based compositors such as Fusion, Nuke, Natron, etc. obviously bypass such layer limitations entirely.

Adobe will hopefully address this at some point. But it would mean re-writing the legacy layer code, and changing the layer mask implementation quite drastically.

The work-arounds include Clipping Masks (as already mentioned) and Knock-Outs in Groups.

Can you provide a screenshot including Layers Panel of a »typical« case in which you would want to use Smart Layer Masks?

Also feel free to add your vote and comments to existing Feature Requests (there may be more, but best stick to ones that already have some votes) …

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What you're looking for is a clipping mask. That's done by placing your Smart Object layer under the one you want to mask.