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

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.)