Nullness - box function from FSharp.Core should return nullable type if argument if nullable and non-nullable type with argument is non-nullable.

[Lanayx]

Repro steps

let y = box 1

Expected behavior

y should be of type obj

Actual behavior

y has type obj | null

Known workarounds

let y = 
    match box 1 with
    | NonNull i -> i
    | Null -> failwith "never fails"

Related information

Provide any related information (optional):

• Windows 11
• dotnet 9.0.0-rc.1.24431.7

[brianrourkeboll]

string null returns "", not null, though.

[Lanayx]

@brianrourkeboll oh, thanks, didn't realize that! Updating the issue

[kerams]

box is not constrained to value types, so obj | null is correct too.

[Lanayx]

@kerams it's is correct with current implementation, but it's not a desired solution, that's why I converted it from bug to feature request :)

[kerams]

It is desired - box can return null:

> box (null: string);;
val it: objnull = <null>

Here's what you want:

let boxv<'a when 'a: struct> (v: 'a) = box v |> Unchecked.nonNull

[vzarytovskii]

Closing per comments, please raise language suggestion for the libraries/language. Currently we can't change non-generic return type of the function based on value.

[Lanayx]

@brianrourkeboll I've just figured out that string can return null, so probably type should be changed (or implementation):

type A() =
    override x.ToString() = null
let y = A() |> string // y is null