FSharpValue.GetUnionFields returns empty array for non-first cases of struct unions with shared fields

[cmeeren]

In F# 9, FSharpValue.GetUnionFields only returns fields for the first case of struct unions with shared fields.

Repro: Repro.zip

open Microsoft.FSharp.Reflection

[<Struct>]
type MySharedStructDu =
    | A of int64
    | B of int64

for value in [A 1L; B 2L] do
    let caseInfo, inner = FSharpValue.GetUnionFields(value, typeof<MySharedStructDu>)
    printfn $"Inner value for case %s{caseInfo.Name} is %A{inner}"

Expected behavior

Inner value for case A is [|1L|]
Inner value for case B is [|2L|]

Actual behavior

Inner value for case A is [|1L|]
Inner value for case B is [||]

Known workarounds

I don't know of any, though to be fair I haven't looked for any either.

[T-Gro]

Thank you for spotting this @cmeeren .
The workaround would be to provide two distinct names to the int64 fields, i.e. reverting to pre-9 behavior (and not making use of the field sharing).

[T-Gro]

F# reflection retrieves the mapping between fields and union cases from IL attributes, fixing this will require emitting additional CompilationMapping attributes.