How does a non optional value obtain a nil value?

Question

The Apple documentation TN2151 says this for a possible cause of a EXC_BREAKPOINT / SIGTRAP:

a non-optional type with a nil valu

Answer 1

Contrived, but possible:

class C {}

let x: C? = nil
let y: C = unsafeBitCast(x, to: C.self)

print(y) // boom

Answer 2

Happens when interoperating with Objective C. An objc-method could be annotated as nonnull, but nothing prevents it from returning nil anyways. The compiler catches only the most obvious cases and even then it only produces warnings (e.g. when you blatently return nil; in a method that is specified as not returning nil.)

Answer 3

Consider something like this in Objective-C bridged to Swift:

- (NSObject * _Nonnull)someObject {
    return nil;
}

The function is annotated as _Nonnull, but will return nil. This bridged as a non-optional object to Swift and will crash.

Answer 4

I Don't know for sure, but maybe this happens if you force-unwrap (!) an Optional into the Non-Optional variable like this:

let a : String? = nil
var x = "hello"
x = a!

But it's just a guess...