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