Extension method on byref can mutate an immutable value

[cartermp]

Consider the following F# code:

open System
open System.Runtime.CompilerServices

[<Extension>]
type Ext() =
    [<Extension>]
    static member inline Change(dt: byref<DateTime>) = dt <- DateTime.Now

let f() =
    let dt = DateTime.Now
    printfn "%d" dt.Ticks
    
    let dtRef = &dt
    
    dtRef.Change()
    printfn "%d" dt.Ticks

    dtRef.Change()
    printfn "%d" dt.Ticks

    dtRef.Change()
    printfn "%d" dt.Ticks

[<EntryPoint>]
let main argv =
    f()
    0 // return an integer exit code

Executing with ctrl-f5:

636676140835468566
636676140835876165
636676140835877191
636676140835878063

[cartermp]

Note that this can also be done with reference types:

open System
open System.Runtime.CompilerServices

[<Extension>]
type Ext() =
    [<Extension>]
    static member inline Change(s: byref<string>) = s <-"Hoopty"

let f() =
    let s = "beef"
    printfn "%s" s
    
    let dtRef = &s
    
    dtRef.Change()
    printfn "%s" s

    dtRef.Change()
    printfn "%s" s

    dtRef.Change()
    printfn "%s" s

[<EntryPoint>]
let main argv =
    f()
    0 // return an integer exit code

Gives

beef
Hoopty
Hoopty
Hoopty

[TIHan]

Highly aware of this. Looking into it.

[zpodlovics]

I guess this is the same or similar problem as the this <- v whole structs replacement here:
fsharp/fslang-design#287 (comment)

Please do not disallow byref access to immutable/read-only structures, as it used for performance critical work (if I remember correctly the readonly/immutable byref changed to inref<'T> to fix this).

[TIHan]

@zpodlovics it looks like it is similar, but those are referring to instance members.

I think I know what's happening in the compiler, but it needs more investigation. Basically on calls like this we lose information that it was a byref or a inref; it only compares the destination type of the byref.

[TIHan]

I have a fix for this coming soon.

[TIHan]

Resolved.