Issue description

When trying to pass a string option value to a null-agnostic method taking string using Option.toObj, the input type gets inferred as string | null option.

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

// assembly A, null-agnostic
let work (x: string) = ()

// assembly B, null-aware
let whatever x =
    work (Option.toObj x)

The type of whatever is inferred to be string | null option -> unit, but string option -> unit seems more reasonable.

Possible workarounds

let whatever (x: string option) =
    work (Option.toObj x)