Private properties no longer break the memberwise initializer in Swift 6.4

SE-0502, shipping in Swift 6.4, changes how the synthesized memberwise initializer treats private stored properties. Previously, a single private property with an initial value would drag the whole initializer's access level down to private, making it unusable from outside the struct. Now such properties are excluded from the initializer instead, keeping it at the highest access level among the remaining properties (capped at internal). The rule requires both that a property be less accessible than the initializer's max access level and that it have an initial value; properties without initial values, or structs where every property is equally private, keep the old behavior. Swift 6.4 also synthesizes a compatibility overload with the old signature to avoid breaking existing same-file calls. The change particularly helps macros that expand to private backing properties, which previously broke memberwise initializers the way property wrappers used to before compiler-level special-casing.

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