Repro. case in F# 4.4.0.0 -

[<CompilationRepresentation (CompilationRepresentationFlags.UseNullAsTrueValue)>]type T = A | B of int | C of int | D of int;;A;;

In F# interactive, here you'll see that A is a proper value, which is not desired.

[<CompilationRepresentation (CompilationRepresentationFlags.UseNullAsTrueValue)>]type T = A | B of int | C of int;;A;;

In F# interactive, here you'll see that A is a null, which is desired.

Would be a nice optimization to have this work for 4 cases as here -https://github.com/bryanedds/Prime/blob/master/Prime/Prime/Prime/Vmap.fs#L25-L30

NOTE: Issue moved from here -fsharp/fsharp#510

Cheers!