Ismissing(x) versus x === missing

[kristoffer.carlsson]

@rikh

So, is it right that ismissing and isnothing is prefered except when you need to squeeze every last bit of performance?

Using === (or !==) allows the compiler to disregard the other alternative type if the inferred type is e.g. a Union{Nothing, T}. Using == or isnothing does not allow this. https://github.com/JuliaLang/julia/pull/38905 might fix this though.