Unscopeable

[Andreas Rossberg]

I'm still trying to wrap my head around the idea of introducing an
@@unscopeable property, as discussed at the last meeting. There seem
to be plenty of edge cases:

* How would it interact with inheritance?

  let p = {a: 1}
  let o = {__proto__: p, [@@unscopeable]: ['a']}
  let a = 2
  with (o) {
    a  // 1 or 2?
  }

  let p = {[@@unscopeable]: ['a']}
  let o = {__proto__: p, a: 1}
  let a = 2
  with (o) {
    a  // 1 or 2?
  }

* How would it affect assignment?

  let o = {[@@unscopeable]: ['a']}
  let a = 2
  with (o) {
    a = 1
    a  // 1 or 2?
    o.a  // 1 or undefined?
  }
  a  // 1 or 2?

  let p = {a: 1, [@@unscopeable]: ['a']}
  let o = {__proto__: p}
  let a = 2
  with (o) {
    a = 3
    a  // 1 or 2 or 3?
    o.a  // 1 or 3?
  }
  a  // 2 or 3?

* How would it interact with mutation?

  let o = {a: 1}
  let a = 2
  with (o) {
    o[@@unscopeable] = ['a']
    a  // 1 or 2?
  }

* How would it interact with compound assignment?

  let o = {a: 1}
  let a = 2
  with (o) {
    a += (o[@@unscopeable] = ['a'], 2)
    a  // 2 or 3 or 4?
    o.a  // 1 or 3?
  }
  a  // 2 or 3 or 4?

* How would it affect the global object? (At least currently, with
scopes and the global scope share the same object environment
mechanism.)

  var a = 1
  this[@@unscopeable] = ['a']
  a  // 1 or undefined?

  var a
  this[@@unscopeable] = ['a']
  a = 1
  a  // 1 or undefined?

  this[@@unscopeable] = ['a']
  </script><script>
  var a = 1
  a  // 1 or undefined?

* How would it expose side effects?

  let s = ""
  let o = {a: 1}
  let u = {get '0'() { s+="0"; return 'a' }, get '1'() { s+="1"; return 'b' }}
  Object.defineProperty(o, @@unscopeable, {get: function() { s += "u";
return u }})

  with (o) {}
  s  // "" or "u" or "u01" or ...?

  with (o) { c; c }
  s  // "u01" or "u01u01" or ...?

  with (o) { a }
  s  // "u01" or "u0" or ...?

  with (o) { a+=1 }
  s  // "u0" or "u00" or "u01" or "u0101" or ...?

Some of this looks rather whacky, if not unpleasant.

/Andreas
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