Nullness issue - ((Type|null):Interface) subsumption in function return values can bypass nullness

[T-Gro]

Issue description

When a function is explicitely annotated to return an instance of an interface, a nullable class implementing can be returned without warnings.
The same error does not occur when subsuming a type with its super type.
The same error does not occur when subsuming an interface with nullable version of the same.

The bug (= i.e. not getting any warning) also occurs when subsuming a nullable version of an interface into a non-nullable parent interface.

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

NET9

Reproducible code snippet and actual behavior

    open System.IO
    open System
    open System.Collections

    let whatisThis (s:Stream|null) : IDisposable =
        s

    let whatisThis2 (s:ICollection|null) : IEnumerable =
        s

Possible workarounds

No response