Why does `@Namespace` not need a type annotation?

I have a question regarding the new @Namespace property wrapper from SwiftUI. In the example code, it is shown without a type-annotation. E.g.:

@Namespace var namespace

How is this possible in Swift? How can this be used for your own types / property wrappers?

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there's only one possible type that wrappedValue can be, which is Namespace.ID so there's no ambiguity what type namespace is

@propertyWrapper struct Foo {
    var wrappedValue: Int = 0
struct Bar {
    @Foo var test1
    @Foo var test2: Int
    // @Foo var test3: String // error: Property type 'String' does not match that of the 'wrappedValue' property of its wrapper type 'Foo'
print(Bar().test1) // prints 0

Thank you. So this is simply an improvement in the type inference algorithm in Swift 5.3?
With Swift 5.2.4 (Xcode 11.5) I get the error-message Type annotation missing in pattern.


I believe this has been the case from the start (when property wrappers were first introduced).


What you're seeing here is default initialization of a property wrapper, which allows the wrapped property type to be inferred when possible. The ability to drop the type annotation from a default-initialized property wrapper was introduced in 5.3 (https://github.com/apple/swift/pull/30663), but this was previously supported by adding empty parentheses after the property wrapper attribute (e.g. @Namespace() var namespace). You can support this in your own property wrappers by providing a default initializer.