Oh no, I'm very certain of this: I distinctly recall a github issue where people complained that addding two 32bit integers resulted in a 64bit integer, which was justified as giving more correct answers due to potential integer overflow.

You're talking about different types of promotion. `Int32 + Int32` did once upon a time give an `Int64` on 64-bit systems. However, that was true regardless of the values of those integers—it was entirely predictable from their types alone. The type of promotion that's being talked about here is promoting `Int + Int` to `BigInt` but only when the values being added are too big to be stored in an `Int`. Julia has never done that.

Ah, ok. I can imagine that that is more JIT-friendly.