Nullness issue - Signature conformance for generic T not validated

[T-Gro]

Issue description

When using .fs and .fsi files, the conformance for generic type parameters and their nullness annotations is not checked.

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

Signature file

[<Class>] 
type GenericContainer<'T when 'T:not null and 'T:not struct> =

    member GetNull : unit -> ('T|null)
    member GetNotNull: unit -> 'T

Impl file:

type GenericContainer<'T when 'T:not null and 'T:not struct>(x:'T) =
    let innerVal = x

    member _.GetNull() : ('T) = x
    member _.GetNotNull() : ('T|null) = null

Possible workarounds

Do not use .fsi files.