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

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...