Is Union{Missing, Float64} a concrete type?

I am optimising part of a small package and I am trying to remove type instabilities. Since I am working with incomplete time series, I am using Union{Missing, Float64} data. I have read online that you want to use concrete types to avoid instabilities. However, I am unsure on whether unions are working fine in this context. Would you please clarify whether this is the case? Reference material / links would also be helpful.

Should not be an issue:

but as always benchmark your concrete use case to see the impact, if there’s a problem we can see whether there’s ways around it.

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Another good (and short) read: Union-splitting: what it is, and why you should care

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Thank you both! It looks like I should not worry about that then.

I will benchmark it with a specialised struct (two entries: float data and missing coordinates) and post the output if the results are interesting.