Looks like the toolchain Foundation doesn't include a method/function that SwiftUI relies on. Apple only guarantees that SwiftUI (which is an Apple-private framework) only works with Apple-sanctioned versions of its other frameworks, like Foundation, which on Darwin may be semi-private (parts open-source, parts proprietary to Apple). Swift development snapshots should be considered beta or alpha software. Most useful for command line tools, may be iffy when trying to mix with the release Apple ecosystem. You can probably get away with using the snapshot compiler, standard library, and the other tools, but frameworks like Foundation are going to be touch and go.
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