Code gen error when repr'ing a type-casted UncheckedArray

[dom96]

type
  Payload = object
    something: int
    vals: ptr UncheckedArray[int]
 
var x: Payload
var vals = [1,2,3]
x.vals = cast[ptr UncheckedArray[int]](addr vals)
x.vals[0] = 42
echo(x.vals[0])
echo(cast[array[5, byte]](x.vals[]).repr)

https://play.nim-lang.org/#ix=3rYy

Output:

/usercode/nimcache/@min.nim.c: In function 'NimMainModule':
/usercode/nimcache/@min.nim.c:208:14: error: assignment to expression with array type
  LOC3.source = x__EGWKGClUlppqAonIP2rgfg.vals;

[ringabout]

Instead cast ptr array

type
  Payload = object
    something: int
    vals: ptr UncheckedArray[int]

var x: Payload
var vals = [1,2,3]
x.vals = cast[ptr UncheckedArray[int]](addr vals)
x.vals[0] = 42
echo(x.vals[0])
echo(cast[ptr array[5, byte]](x.vals)[].repr)