Swift compiler failing with incorrect error when initialising @State variable

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Created Apr ’23

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We have a large project where we have some View's that are being passed data in their initialisers to setup various @State variables. Recently we added some code to one of these views and the compiler immediately failed to compile it, indicating a compilation failure on a line that had not been changed. After some work to understand the problem we distilled it down to the following examples.

Here is some original code that compiles, it sets the State variable in the init() because that matches the much bigger source this was distilled from.

struct SomeView: View {
    @State var anInt: Int
    init() {
        anInt = 5.
    }
    var body: some View {
        Text("Hello \(anInt)")
    }
}

Now we add a Date variable and the compile fails with an error:

struct SomeView: View {
    @State var anInt: Int
    let date = Date()           &#x2F;* New variable declaration. *&#x2F;
    init() {
        anInt = 5               &#x2F;* Compile fails with error: Variable &#039;self.anInt&#039; used before being initialized *&#x2F;
    }
    var body: some View {
        Text("Hello \(anInt)")
    }
}

This obviously caused much confusion here. Now at this point we managed to fix the code by doing this:

struct SomeView: View {
    @State var anInt: Int
    let date = Date()           &#x2F;* New variable declaration. *&#x2F;
    init() {
        _anInt = State(initialValue: 5)
    }
    var body: some View {
        Text("Hello \(anInt)")
    }
}

However it doesn't explain why the compilation failed in the first place. Nor is it great as it feels like we're going backwards when it's perfectly ok to just set anInt = 5 with the latest compilers.

We'd like, if possible, for a Swift compiler engineer to take a look and give us an explanation (so we can better understand the issue) and possible improve the error reporting in future compiler releases. Currently we think it's something to do with Date being a ReferenceConvertable as swapping in other ReferenceConvertable's also trigger the same compiler error.

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Answer 1

Claude31 OP

Apr ’23

why the compilation failed in the first place.

That's because init() is called at first, even before let date is initialised. Hence the error. It would be difficult to change, as it involves the whole initialisation process

But using a computed var does work:

struct SomeView: View {
    @State var anInt: Int
    &#x2F;&#x2F;    let date = Date()           &#x2F;* New variable declaration. *&#x2F;
    var date : Date {
        Date() 
    }

    init() {
        anInt = 5               &#x2F;* Compile fails with error: Variable &#039;self.anInt&#039; used before being initialized *&#x2F;
    }
    var body: some View {
        Text("Hello \(anInt)")
    }
}

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