Nullness issue - Unsafe cast requires generic type to be non-null

[Lanayx]

Issue description

I suggest that unsafe cast shouldn't impose any restrictions on 'T, it can be nullable, non-nullable or any other type, it should be fully generic. There is nothing in initial code about nullables, so it's very confusing to user to see such error.

The build currently works for me in 9.0.1 SDK, but breaking change is coming according to this ticket https://youtrack.jetbrains.com/issue/RIDER-121095/Non-existing-NRT-related-errors

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

.NET SDK 9.0.101

Reproducible code snippet and actual behavior

let test<'T> () =
    let t = obj()
    t :?> 'T

Possible workarounds

let test<'T> () =
    let t: obj | null = obj()
    t :?> 'T

or

let test<'T> () =
    let t = obj()
    t |> unbox<'T>

[T-Gro]

I think the line of design thinking was:

If the source of the upcast is known to be not-null, then the target can be not-null as well.
If the source of the upcast is possibly null, the target is also possible null.

(excl. specialties like NullAsTrueValue used for option etc.)

Is it more about the usability of not having to spell out the constraint, or would you indeed want to call such an API with a 'T|null` type?

[Lanayx]

This was my initial case

static member GetService<'T>(ctx: HttpContext) =
      let t = typeof<'T>
      match ctx.RequestServices.GetService t with
      | null -> raise <| Exception t.Name
      | service -> service :?> 'T

So I didn't know how to fix this without introducing breaking change to consumers (because adding constraint would have introduced breaking change). After some thought I did the breaking change anyway, since in this particular case logic really didn't allow null to go through.

As for usability - this is definitely the problem as well, imagine a beginner stumble upon the code from the issue description, they won't have any idea that the issue lies in nullability.

So, for my particular case the usability impact was stronger. I think that it would make sense to fix usability first and revert the behavior if more cases justify that.