Nullness issue - Misleading error or incorrect type inference

[Lanayx]

Issue description

Currently compiler can't unify null and string into single string | null type and gives misleading error in the place where there are no nulls:

Program.fs(2, 24): [FS3261] Nullness warning: The type 'string' does not support 'null'.

Ideally compiler would be able to unify cases, or provide better error message, or underline the null case rather than the non-nullable string.

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 10.0.202

Reproducible code snippet and actual behavior

let test x =
    match x with
    | null -> ()
    | "" -> ()
    | _ -> ()

Possible workarounds

Manually specify type

let test (x: string | null) =
    match x with
    | null -> ()
    | "" -> ()
    | _ -> ()

[vzarytovskii]

I don't think those should be unified. Nullness should be explicit, since we don't want encourage it.