Optional Type Casting Issues

OptionalTypeCastingIssues.swift

class Foo {
    
}

class Bar: Foo {
    
}

//let b: Bar? = nil
let b: Bar? = Bar()
let f: Foo? = b

if b is Foo {
    print("Foo")
}

if b is Foo? {
    print("Foo?")
}

if b is Bar {
    print("Bar")
}

if b is Bar? {
    print("Bar?")
}

switch b {
case is Foo:
    print("Foo")
    
case is Foo?:
    print("Foo?")
    
case is Bar:
    print("Bar")
    
case is Bar?:
    print("Bar?")
    
default:
    break
}

This code snippet reveals so many issues:

1/ Playground reports this warning:

13:6: warning: cast from 'Bar?' to unrelated type 'Foo' always fails```
if b is Foo {
~ ^ ~~~

But REPL executes the same code successfully:

14> if b is Foo {
15. print("Foo")
16. }
Foo

2/ Playground reports this error:

21:6: error: downcast from 'Bar?' to 'Bar' only unwraps optionals; did you mean to use '!'?
if b is Bar {
~ ^ ~~~

But as you just saw above, b is Foo is true. Why can't we test b is Bar?

3/ Playground reports this warning:

17:6: warning: 'is' test is always true
if b is Foo? {
^

But it is not true. After commenting out the code that triggers the 2nd error above, it changes to this runtime exception:

Could not cast value of type 'Swift.Optional' (0x1125ad238) to 'Swift.Optional' (0x1125ad448).

Why can't? Why line 11( let f: Foo? = b) can?

4/ After commenting out the code that triggers the runtime exception above, all the playground warnings go away.

5/ After commenting out the first 2 cases in switch, the 3rd case evaluates to true:

case is Bar:
    print("Bar")

But do you remember the 2nd error above? It does't allow us to test if b is Bar. Why case is Bar is allowed?