Issue description

type R = { x: int }
type RA = {| x: int |}

[<EntryPoint>]
let main _args =
    let a = System.Text.Json.JsonSerializer.Deserialize<{| x: int |}> "null"
    let _a = a.x

    let b = System.Text.Json.JsonSerializer.Deserialize<RA> "null"
    let _b = b.x

    let c = System.Text.Json.JsonSerializer.Deserialize<R> "null"
    let _c = c.x
    0

The return type of Deserialize is 'T | null, but a warning is only raised for the nominal record on c.x, yet a.x and b.x will throw too.

Additionally, there's a discrepancy in tooltips - a is val a: {| a: int |}, but on b, where the type is aliased, it is val b: RA | null.

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.2.24474.11

Reproducible code snippet and actual behavior

No response

Possible workarounds

No response