copyOfStruct error when setting a property returned from a function

[mrakgr]

See also #8069 which is being treated as a duplicate of this

module Bug

type Q() =
    member val t = 0 with get, set

let f c x = c () <- x

let q = Q()
f (fun _ -> q.t) 2

1>C:\Users\Marko\Source\Repos\Lithe\Lithe\bug.fs(6,13): error FS0971: Undefined value 'copyOfStruct : 'a ref'

This only gives me an error at build time, not in VS.

[abelbraaksma]

From #8069 there comes this similar repro, which should find its way to tests once this gets picked up:

[<EntryPoint>]
let main argv = 

    let mutable way1 = [|0|]
    let mutable way2 = [|0|]
    (way1,way2) <- ([|1|],[|2|])   // FS0971 copyOfStruct error after compiling
    printfn "%A" argv
    0 

[ForNeVeR]

Less arcane way of reproducing the issue:

open System
type C() =
    member _.M() =
       let mutable a, b = 3, 4
       (a, b) <- 4, 5 // error FS0971: Undefined value 'copyOfStruct : (int * int) ref'
       a, b

sharplab

[abelbraaksma]

@ForNeVeR, interesting. Previously, assignment to a tuple led to a different error iirc. After all, a tuple is immutable (regardless of the declaration, which is short for two distinct declarations of variables), so an error makes sense, but not that one.