Weak reference doesn't work as expected when passing it as a method reference

I am already aware of the strong/weak reference concept in swift.
yet after running the next code, and tapping on the Button (and dismissing the screen), the TestViewModel stayed in memory!
I was expecting that using [weak viewmodel] will be enough to prevent it.
in the second example I managed to fix it - but I don't understand why it worked

import SwiftUI
import Resolver

struct TestScreen: View {
    
    @StateObject var viewmodel = TestViewModel()
    @Injected var testStruct: TestStruct
    @Environment(\.presentationMode) var presentationMode
    
    var body: some View {
        NavigationView {
            
            VStack(spacing: 0) {
                
                Button("go back") { [weak viewmodel] in
                        testStruct.saveActionGlobaly(onAsyncAction:  viewmodel?.someAsyncAction )
                        presentationMode.wrappedValue.dismiss()
                    }
            }
        }
    }
}

import Foundation
import Resolver
import SwiftUI

public class TestStruct {
   var onAsyncAction: (() async throws -> Void)?
    
    public func saveActionGlobaly(onAsyncAction: (() async throws -> Void)?) {
        self.onAsyncAction = onAsyncAction
    } 
}

EXAMPLE 2:
I managed to prevent the leak by changing the code this way:
(notice the changes in the callback passed to onAsyncAction)

import Resolver

struct TestScreen: View {
    
    @StateObject var viewmodel = TestViewModel()
    @Injected var testStruct: TestStruct
    @Environment(\.presentationMode) var presentationMode
    
    var body: some View {
        NavigationView {
            
            VStack(spacing: 0) {
                
                Button("go back") { [weak viewmodel] in
                        testStruct.saveActionGlobaly(onAsyncAction:  { await viewmodel?.someAsyncAction() } )
                        presentationMode.wrappedValue.dismiss()
                    }
            }
        }
    }
}

I dont understand why the second TestScreen managed to apply the weak reference and the first one didn't,
thanks (:

environment:
swift 5
xcode 14.2

Your first version:

testStruct.saveActionGlobaly(onAsyncAction:  viewmodel?.someAsyncAction )

is equivalent to this:

let action: (() async throws -> Void)?
if let vm = viewmodel {
    // vm is a strong non-nil reference, so this closure
    // has a strong non-nil reference to a TestViewModel.
    action = vm.someAsyncAction
} else {
    action = nil
}
testStruct.saveActionGlobaly(onAsyncAction: action)

SwiftUI holds on to your @StateObject for as long as TestScreen is part of the view hierarchy, which is as long as the Button is part of the view hierarchy. So SwiftUI maintains a strong reference to your TestViewModel until after it has called your Button's action. So in your first version, your weak viewmodel reference inside the Button's action will never be nil. Therefore vm will never be nil, action will never be nil, and action will always have a strong reference to the TestViewModel.

Your second version:

testStruct.saveActionGlobaly(onAsyncAction:  { await viewmodel?.someAsyncAction() } )

preserves the weakness of the viewmodel variable. It only creates a strong reference to the TestViewModel momentarily, each time it is invoked, and discards the strong reference immediately after someAsyncAction returns.

Using rob's answer and doing some extra reading I think I managed to shed some more light over this (at least for me, as rob's answer is ofcourse correct):
first of all, the weak-reference concept is a compiler game - meaning, the complier runs over the first example and translate it to: (as rob described)

let action: (() async throws -> Void)?
if let vm = viewmodel {
    // vm is a strong non-nil reference, so this closure
    // has a strong non-nil reference to a TestViewModel.
    action = vm.someAsyncAction
} else {
    action = nil
}
testStruct.saveActionGlobaly(onAsyncAction: action)

which makes sense...

the missing part for me was understanding the next sentence by rob:

> action will always have a strong reference to the TestViewModel

so, there is another step where the compiler translate action to be a closure like so (very abstract):

{
   viewmodel.action // implicit viewmodel
}

and hands it over to onAsyncAction argument.
in other words, the returned closure from evaluating action is holding another implicit viewmodel reference. the compiler can't conclude the explicit and the implicit viewmodels are related, thus the weakness is not applied to the later