Nullness issue - Signature conformance of .fsi nullness and infered nullness from .fs

[T-Gro]

Issue description

A nullness warning is missing when an .fsi specifies strict non-nullable type and implementation uses type inference to the nullable version of the same type

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 9

Reproducible code snippet and actual behavior

Signature

val iRejectNulls: string -> string

Implementation

let iRejectNulls (x:_|null) = 
    match x with
    | null -> "null"
    | s -> String.length s |> string

Possible workarounds

Do not use .fsi