copyOfStruct error when setting a property returned from a function

See also https://github.com/dotnet/fsharp/issues/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.