add union-splitting fallback for accepted_field_type

[jrevels]

julia> Legolas.@schema "example" Example

julia> Legolas.@version ExampleV1 begin id::Union{UUID,Missing} end

julia> Legolas.accepted_field_type(ExampleV1SchemaVersion(), UUID)
Union{UUID, UInt128}

julia> Legolas.accepted_field_type(ExampleV1SchemaVersion(), Union{UUID,Missing})
Union{Missing, UUID}

I have to explicitly overload accepted_field_type for Union{UUID,Missing} if I want to capture the UUID -> Union{UUID,UInt128} fallback

Though it's not incorrect at all, needing to do this is annoying and unintuitive

It'd be nice to have an accepted_field_type fallback that automatically splits union types, applies the accepted_field_type transform to each constituent type, and then re-unions the constituents

[jrevels]

I wonder whether it's also worth implementing a similar fallback for eltypes of Arrow.jl-supported container types (Vector, Tuple, etc.) which basically exhibit this same issue

[jrevels]

@omus called out (or, i inferred some code he wrote 😁 ) that invoke may also be useful here in some cases, e.g.:

function accepted_field_type(sv::Legolas.SchemaVersion, ::Type{T}) where T >: Missing
     A = accepted_field_type(sv, nonmissingtype(T))
     return Union{A,Missing}
end

the fully generic one might require general union-splitting though

[iamed2]

Julia 1.9+ now has typesplit which could be used for generic union-splitting (not exported unfortunately, but better than Core.Compiler.typesubtract)

nonmissingtype is just nonmissingtype(::Type{T}) where {T} = typesplit(T, Missing) now