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We already have solutions to both problems, with traits and class inheritance.
I see no immediate benefit of the proposed solution over the userland implementations. The RFC mentions a shopping cart example, but I don't think that's cleaner than using league/pipeline or Laravel's pipeline.
It's a bit messy for the simpler examples as well.
It's almost as messy as putting all the functions into each other.
The idea is a nice one, and one that I would welcome, but this proposal puts forward messy syntax that isn't clear!