about shows the wrong representation for small unions (Julia doesn't expose the needed information)

[NHDaly]

Here is an example of a small union in a struct.

For isbits types, this does not turn into a pointer. Instead, what happens is that the Union is essentially expanded into a tuple of (T, Int8), where the Int8 is a type tag that indicates whether y is a Nothing or a T.

But if T is a mutable value, y is going to need a pointer for T anyway, and so it can use only a single pointer for the whole union. If the pointer points to the singleton address for nothing, y is a Nothing, otherwise it's a T.

This basically compiles down to a "null-check" on the pointer, but instead of checking for "null," it checks for the singleton address for "nothing," which could be e.g. 0xaabbc123 or something.

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Instead, this currently incorrectly marks the union as always a Ptr:

julia> struct MyStruct32{T}
           x::Int32
           y::Union{T,Nothing}
       end

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However, unfortunately, I think that currently julia doesn't expose enough information to answer this correctly. I'm not positive, but i think so.
See this feature request to expose it: JuliaLang/julia#44943 (comment)