Fun. Basically, it decides that (due to UB) it can simply not materialize the argument and carry on.
On Darwin/x86_64, this actually means that it prints out bits from an "uninitialized" register, specifically %rsi in this case, since that's where the second argument should be (even for variadic functions).
Certainly a jarring failure mode! However, if updating your compiler causes you to run into this, you already have a bug in your code (one that clang -- arguably not forcefully enough -- already warns you of).
