This is bug or not? (immutable class containing struct with dtor)

[Denis Feklushkin via Digitalmars-d-learn]

```d
/+ dub.json:
{
  "name": "test",
  "dependencies": {
  }
}
+/
struct S
{
    ~this() {}
}

immutable class Imm
{
    S s; // this is immutable value because whole class is 
immutable

    this()
    {
        s = S();
    }

    ~this() {} // Comment out this to fix this compilation  error:
    // Error: `immutable` method `serializer_bug.Imm.~this` is 
not callable using a mutable object
    // What mutable object is meant here?
}


void main()
{
    auto ic = new immutable Imm();
}
```

Run: $ dub --single bug.d