Nullness issue - Spurious nullness warning with flexible type

[glchapman]

Issue description

I get a nullness warning with the following:

let dispose (x: IDisposable | null) : unit =
    match x with
    | null -> ()
    | d -> d.Dispose()

let useThing (thing: #IDisposable) =
    try
        printfn "%O" thing
    finally
        dispose thing // warning generated

Since the dispose function allows null, I do not think a warning should be generated. No warning is generated for a non-flexible type, e.g.:

let useThing (thing: IDisposable) =
    try
        printfn "%O" thing
    finally
        dispose thing // no warning generated 

Choose one or more from the following categories of impact

• Unexpected nullness warning (false positive in nullness checking, code uses --checknulls and langversion:preview).
• Missing nullness warning in a case which can produce nulls (false negative, code uses --checknulls and langversion:preview).
• Breaking change related to older null constructs in code not using the checknulls switch.
• Breaking change related to generic code and explicit type constraints (null, not null).
• Type inference issue (i.e. code worked without type annotations before, and applying the --checknulls enforces type annotations).
• C#/F# interop issue related to nullness metadata.
• Other (none of the categories above apply).

Operating System

Windows (Default)

What .NET runtime/SDK kind are you seeing the issue on

.NET SDK (.NET Core, .NET 5+)

.NET Runtime/SDK version

net9.0

Reproducible code snippet and actual behavior

No response

Possible workarounds

No response

[glchapman]

I just reread the section "Inference in generic code" from Nullable Reference Types in F# 9. I thought perhaps it meant that the following should work without a warning, but a warning is still generated

let useThing<'d when 'd: not struct and 'd :> IDisposable> (thing: 'd) =
    try
        printfn "%O" thing
    finally
        dispose thing // warning generated

[T-Gro]

Could you please also include which warning this produces incl. the range (= coordinates in code) for it?

[glchapman]

The warning is:

Program.fs(12,17): warning FS3261: Nullness warning: The types 'IDisposable' and 'IDisposable | null' do not have equivalent nullability.

In VSCode, thing in the last line (dispose thing) is highlighted.