VM fails when returning var char via var string

[metagn]

Example

# fine:
proc access(s: var seq[char], i: int): var char = s[i]

static:
  var s = @['a', 'b', 'c']
  access(s, 2) = '!'
  discard access(s, 0)

# fails:
proc access(s: var string, i: int): var char = s[i]

static:
  var s = "abc"
  access(s, 2) = '!'
  discard access(s, 0)

Current Output

(14, 9) Error: attempt to access a nil address
if you comment out the previous one:
(15, 17) Error: attempt to access a nil address kind: rkInt

Expected Output

compiles

Additional Information

• possibly related: nimvm: CritBitTree does not work: "limited VM support for 'addr'" #14339, VM and js backend require result for lent or var annotations to work #14420

$ nim -v
Nim Compiler Version 1.3.5
(latest devel)

[treeform]

Maybe related? #16226