Warning FS3560 triggered upon nested update even if other fields of nested record are copied

[reinux]

Repro steps

type Q = { x: int; y: string }
type T = { q: Q }
let a = { q = { x = 5; y = "abc" } }
let b = { a with q.x = 5 }

Expected behavior
No warning

Actual behavior

"This copy-and-update record expression changes all fields of reecord type 'T'. Consider using the record construction syntax instead.

Known workarounds
<NoWarn>FS3560</NoWarn> in .fsproj

Related information

• Operating system: Linux (NixOS Unstable)
• .NET Runtime kind: .NET Core
• Editing Tools: Rider 2023.3.4

[edgarfgp]

I'm not able to reproduce this on main with withLangVersion80 or withLangVersionPreview.

Edit: Adding some extra tests for this could be useful

Edit 2: Looks like this warning is disabled by default(It was not like that in the past).

Anyway. Yeah it can be reproduced. See:

[<Fact>]
let ``Warning FS3560 triggered upon nested update even if other fields of nested record are copied`` () =
        Fsx """
type Q = { x: int; y: string }
type T = { q: Q }
let a = { q = { x = 5; y = "abc" } }
let b = { a with q.x = 5 }
        """
        |> withLangVersion80
        |> withOptions ["--warnon:FS3560"]
        |> typecheck
        |> shouldFail
        |> withDiagnostics [
            (Warning 3560, Line 5, Col 9, Line 5, Col 27, "This copy-and-update record expression changes all fields of record type 'Test.T'. Consider using the record construction syntax instead.")
        ]

[kerams]

Why do you think this is a bug?

[brianrourkeboll]

Why do you think this is a bug?

Yeah, I guess technically the message is correct in that

{ a with q.x = 5 }

could be

{ q = { a.q with x = 5 } }

But I can also see why someone would view copying-and-updating the two nested records in this case as a single copy-and-update operation (now that that's possible), rather than as an outer construction operation and an inner copy-and-update operation.

[abonie]

We should remove this warning and do what the warning says during code-gen or optimization

[abonie]

Closing, since we plan to address it in a different manner. Btw this warning is disabled by default as Edgar said (except in some old version of the SDK)