Add Unchecked.withNull for unconstrained C# nullable-generic interop

[T-Gro]

Relates to #17734.

Adds Unchecked.withNull : 'T -> 'T | null, an escape hatch for implementing and consuming unconstrained C# nullable-generic APIs (T? M<T>(), interface { T? GetValue<T>(int) }). Unlike Operators.withNull it places no not null/not struct constraint on 'T, so such a slot can be satisfied without FS3261.

It is inline and erases to a retype ((# "" value : 'T | null #)) — nothing lands in IL. Documented as unsafe/interop-only: 'T | null is meaningless for a struct 'T. Assigning null to a resulting mutable at a struct lowers to default(T) (ILVerify-clean through multiple generic layers); the same on a concrete struct mutable stays a compile error (FS0043).

cc @Lanayx @jwosty

[github-actions]

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`src/FSharp.Core`	docs/release-notes/.FSharp.Core/11.0.100.md

[Lanayx]

@T-Gro I'm currently on vacation, can't check properly, but will this work for wrapped cases? Like for Task<'T | null>. If not, this helper might be of little use.

[T-Gro]

will this work for wrapped cases? Like for Task<'T | null>.

Yes. It works for the wrapped case.

You make the nullable value with withNull in generic code. You give the value to Task.FromResult. The return type becomes Task<'T | null>. This meets the Task<T?> slot. The generic member keeps no not null or not struct constraint.

{ new IAsyncEventContext with
    member _.GetValueAsync<'T>(index) =
        Task.FromResult(Unchecked.withNull (Unchecked.defaultof<'T>)) }  // : Task<'T | null>

Note: for a struct type, you cannot read the result back at a fixed type. F# reports FS3265 there. This is correct. A struct has no null value.

I added two tests for this.

[jwosty]

This is great, I just wonder if there might be a better name out there? At the risk of bikeshedding: I have a feeling that future me is going to be prone to forgetting Unchecked.withNull is. Just spitballing: Unchecked.asUnconstrainedNullable, Unchecked.withNullUnconstrained, Unchecked.asNakedNull. Plus a long name would discourage people from using it willy-nilly (a good thing). Super minor though -- all good if there isn't a better name than Unchecked.withNull

Thanks for the rapid action Tomas!