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

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.