What kind of magic behavior is this?

Using Swift
25

There are long standing compiler bugs in this area.

For example the following (AFAICS clearly invalid) program still compiles with Swift 4.2 (haven't checked 5.0):

struct X<T> {
    var a : A
}
extension X where T == Never {
    typealias A = Int
}
let x = X<Int>(a: 123)