XO0503: one variable or overlapping places passed to two `var` parameters of one call

A call may not pass the same variable, or two places that overlap (x and x.f, xs[i] and xs[j] unless both indexes are different literals, a var receiver and an argument naming it), to two var parameters. The Go backend passes var arguments by reference and the native backends copy them in and out, so aliased writes would differ; with aliasing ruled out both conventions give one result. Copy one of the places into a temporary, pass the temporary, and store it back after the call, or take a single var parameter of a struct that holds both values. See core 5.2 and decision 0111.

Example

fn both(var a: List[Int], var b: List[Int]) {
  a.push(1)
  b.push(2)
}

fn f() -> Int {
  var xs = [0]
  both(xs, xs)
  xs.len()
}

Fix

fn both(var a: List[Int], var b: List[Int]) {
  a.push(1)
  b.push(2)
}

fn f() -> Int {
  var xs = [0]
  var ys = xs
  both(xs, ys)
  xs.len() + ys.len()
}

Run xo explain XO0503 for this text in a terminal, or see section 10.6 of the specification.