Nullness issue - matching null with an int must be an error, not just a warning

[T-Gro]

Issue description

Code which uses generic constructs that require null can lead to incorrect IL when used with the checknulls+ flag.
With this flag, it becomes "just" a warning, but such code must not compile and should be an error instead.

This is particulary a must when unifying a generic type that has the null (or more generally, not struct) constraint with a value type like int in this example.

let test = 
    match null with
    | null -> true
    | 42 -> false

Produces:
warning FS3261: Nullness warning: The type 'int' does not support 'null'
warning FS0025: Incomplete pattern matches on this expression. For example, the value '0' may indicate a case not covered by the pattern(s).

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

No response

Reproducible code snippet and actual behavior

let test = 
    match null with
    | null -> true
    | 42 -> false

Possible workarounds

• Use warnings as errors
• Ignore this