Can the compiler sometimes optimize small unions to avoid boxing?

Hi, random question about performance: can the compiler sometimes optimize Union type variables so that there is no boxing or unboxing? For example, if I declare an “optional” integer type like Union{Nothing, Int}would the compiler be smart enough to keep that on the stack?

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Yes, if the small union is type-stable and compiled, then this is guaranteed. It’s actually what powers iteration — so it really needs to be fast — and is why small unions like these are flagged in yellow by @code_warntype instead of red. The compiler just drops this down to a simple if statement with two unboxed branches.