[ARC] SIGSEGV with tuple assignment caused by cursor inference

[ghost]

The bug in #15110 was fixed, but turns out there's another ARC bug affecting https://github.com/PMunch/combparser.

Example

type
  Maybe = object
    value: seq[int]

proc p1(): Maybe =
  let lresult = @[123]
  var lvalue: seq[int]
  var lnext: string
  (lvalue, lnext) = (lresult, ";")
  
  result.value = lvalue

proc main = 
  doAssert p1().value == @[123]

main()

Current Output

Traceback (most recent call last)
/home/dian/Stuff/arctest/combparser/src/combparser.nim(23) combparser
/home/dian/Stuff/arctest/combparser/src/combparser.nim(11) main
/home/dian/Things/Nim/lib/system/alloc.nim(972) dealloc
/home/dian/Things/Nim/lib/system/alloc.nim(871) rawDealloc
/home/dian/Things/Nim/lib/system/avltree.nim(74) del
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Expected Output

No SIGSEGV, assertions passed

$ nim -v
Nim Compiler Version 1.3.5 [Linux: amd64]
Compiled at 2020-07-30
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: f1971243d3e162889b10b46caab8ba6e98afa92d
active boot switches: -d:release -d:danger

[ghost]

Seems to work when cursor inference is disabled.
expandArc for p1

var
  lresult
  lvalue
`=`(lresult, [123])
var lnext_cursor: string
let __cursor = (lresult, ";")
lvalue = __cursor[0]
lnext_cursor = __cursor[1]
`=sink`(result.value, lvalue)
`=destroy`(lresult)

for ret:

`=sink`(result.value, input[0 .. 1])