gg.nim

type
  Maybe = object
    value: string

proc charmatch(input: string): Maybe =
  # If you replace this line with just input
  # or something else - it works.
  result.value = input[0 .. ^1]

proc p1(): Maybe =
  let lresult = charmatch("123;")
  # The line below is needed - if you use var/let directly in 
  # lvalue, lnext assignment - it works.
  var lvalue, lnext: string
  (lvalue, lnext) = (lresult.value, ";")
  
  result.value = lvalue

proc main = 
  let r1 = p1()
  doAssert r1.value == "123;"

main()