Nullness issue - passing nullable value as non-nullable method parameter works without warning

[Lanayx]

Issue description

I expect that passing nullable value as non-nullable method parameter should trigger warning, but it doesn't

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

9.0.100-rc.1.24452.12

Reproducible code snippet and actual behavior

type Test =
    static member X(o: obj) = ""

let x: obj | null = null
let y = Test.X x // warning expected

Possible workarounds

No workaround is needed

[T-Gro]

Thanks for this report, this is indeed a serious bug.
In the beginnings of this feature, obj has been left out of nullness checks, but that should not be the case anymore.