Issue description

Consider this code as a placeholder for a type defined in an external library:

[<AllowNullLiteral>]
type IInterfaceFromExternalPackage = interface end

[<AllowNullLiteral>]
type ImplementorFromExternalPackage() =
    interface IInterfaceFromExternalPackage

Now consider the following:

open Microsoft.Extensions.DependencyInjection

ServiceCollection().AddSingleton<IInterfaceFromExternalPackage>(ImplementorFromExternalPackage())

This causes a compiler warning:

FS3261 : Nullness warning: The type 'IInterfaceFromExternalPackage' supports 'null' but a non-null type is expected.

Of course, there is no actual nullness issue here since I pass a non-null (singleton) instance of the type. However, I can find no way of silencing this warning apart from disabling it entirely for the file.

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 SDK 9.0.100

Reproducible code snippet and actual behavior

No response

Possible workarounds

No response