[nem-en] Pattern matching using `is' loses bindings

[Snaury]

Hi everyone,

I'm not sure whether it is a bug (because I think I understand why
this happens this doesn't seem to be a bug) and needs to be fixed, but
it's something that annoys me sometimes. Consider the code:

  def foo() : option[int * int] {
    Some((51, 67))
  }

  when(foo() is Some((a, b)))
    System.Console.WriteLine("({0},{1})", a, b);

Here I use `is' pattern matching inside of `when', because there are
situations when it's too expensive (in terms of typing) to write full
match. Variables a and b are reported unbound here, and I think I
understand why: because the scope of a and b is inside of condition of
`when', and when `when' gets result it leaves that scope, thus a and b
just disappear. However, there are situations (in complex match cases)
when this behaviour beats hard into my back:

  def foo(i) : option[int * int] {
    if(i > 20)
      Some((i * 2, i * 3))
    else
      None()
  }

  def fee(i) : option[int * int] {
    if(i < 10)
      Some((i * 4, i * 2))
    else
      None()
  }

  match(18 /* or 35, 8, etc */) {
    | x when (x > 10 && foo(x) is Some((a, b))) =>
      System.Console.WriteLine($"x > 10 leads to ($a,$b)");
    | x when (x > 5 && fee(x) is Some((a, b))) =>
      System.Console.WriteLine($"x < 5 leads to ($a,$b)");
    | x =>
      System.Console.WriteLine($"x = $x");
  }

Here `when' in this (relatively simple) match is not that easy to
refactor into another match, because we will have to match None/_ case
and thus would never jump to the next case. I'm not sure if anything
can be done with matching inside of when with variable bindings, but
if only it could be improved in some way...