How case works in if-case

The operator is if case, so you can’t put parentheses. The syntax and behavior are based on those of the case statement in a Swift switch statement (see my online book if you need details). In a case statement, 20...30 is an interval, used as a pattern, which operates by using contains against the interval. The equals sign is indeed truly confusing, but that was their first attempt at a syntax for expressing what the case statement should be comparing with (i.e. the tag that comes after the switch keyword in a switch statement).

So, if you understand this:

switch age {
case 20...30:
    // do stuff
default:break
}

… then you understand how it is morphed directly into this:

if case 20...30 = age {
   // do stuff
}