extension Int: Factory {}でIntに特化されたやつ
// thunk sil shared [transparent] [reabstraction_thunk] @reabstraction thunk helper from @callee_owned () -> (@unowned Swift.Int) to @callee_owned () -> (@out Swift.Int) : $@convention(thin) (@owned @callee_owned () -> Int) -> @out Int { // %0 // user: %3 // %1 // user: %2 bb0(%0 : $*Int, %1 : $@callee_owned () -> Int): %2 = apply %1() : $@callee_owned () -> Int, scope 7 // user: %3 store %2 to %0 : $*Int, scope 7 // id: %3 %4 = tuple (), scope 7 // user: %5 return %4 : $(), scope 7 // id: %5 } // end sil function 'reabstraction thunk helper from @callee_owned () -> (@unowned Swift.Int) to @callee_owned () -> (@out Swift.Int)'