I have a scenario like this:

function TestStruct() constructor {
    prop1 = 1;
    prop2 = 2;
}

function Wrapper() constructor {
    contents = {
        a : new TestStruct(),
        b : new TestStruct()
    }
}

and if I add some extra definitions to the contents struct anywhere in the project (like below, where a custom property is passed via a constructor argument)

function Wrapper(myCustomProperty) constructor {
    contents = {
        a : new TestStruct(),
        b : new TestStruct()
    }

    contents[$ myCustomProperty] = new TestStruct();
}

I get a warning at the closing brace of the contents struct which says:

Can't cast { a: TestStruct, b: TestStruct } to array<?> for assignment

It is less of a major inconvenience, but more so an oddity that leaves me with a warning for what appears to me to be correct code.