collect leaking scope, and maybe another bug

[nblaxall]

This was tested on https://play.nim-lang.org/#ix=3Jnf and also on my laptop (Nim Compiler Version 1.6.2 [MacOSX: amd64])

Sugar's collect seems to leak scope, plus maybe another bug to do with if/else's in collect:

Example

import std/sugar

let a = collect:
  let n = 2
  for i in 1..9:
    n * i
echo a  # @[2, 4, 6, 8, 10, 12, 14, 16, 18]

echo n  # 2 ... leaks out of scope :(

let b = collect:
  #let n = 1  # Error: redefinition of 'n' :(
  let m = 2
  for i in 1..9:
    m * i
echo b  # @[2, 4, 6, 8, 10, 12, 14, 16, 18]

let c = collect:
  let o = 2
  for i in 1..9:
    if o == 2: o
    else: 0  # works
echo c  # @[2, 2, 2, 2, 2, 2, 2, 2, 2]

let d = collect:
  let p = 2
  if p == 2:
    for i in 1..9: i
  else:
    for i in 1..9: i * p  # breaks
echo d  # expected should be: @[1, 2, 3, 4, 5, 6, 7, 8, 9]

here is the output of https://play.nim-lang.org/#ix=3Jnf:

Hint: used config file '/playground/nim/config/nim.cfg' [Conf] Hint: used config file '/playground/nim/config/config.nims' [Conf] ............................................................. /usercode/in.nim(25, 9) template/generic instantiation of collect from here /usercode/in.nim(30, 22) Error: expression 'i * p' is of type 'int' and has to be used (or discarded)

[ghost]

Scope leakage fixed in https://github.com/nim-lang/Nim/pull/19288

About this:

let d = collect:
  let p = 2
  if p == 2:
    for i in 1..9: i
  else:
    for i in 1..9: i * p  # breaks

I'm not really sure if this is supposed to work as-is. You can make it work by changing the code so that if is inside the loop:

let d = collect:
  let p = 2
  for i in 1..9:
    if p == 2: 
      i
    else:
      i * p