Error: ignored invalid for loop for it triggered when changing a # to a ##

[timotheecour]

import tables

when defined(case1):
  # Error: ignored invalid for loop for it {.inject.} in s2:
  template items*[A,B](t: Table[A,B]): tuple[key: A, val: B] =
    ## foobar
    pairs(t)

when defined(case2):
  # ok
  template items*[A,B](t: Table[A,B]): tuple[key: A, val: B] =
    # foobar
    pairs(t)

when isMainModule:
  import sequtils
  let a = initTable[int, string]()
  let b = mapIt(a, it.key)

this prevents the simpler suggestion in #9217 (comment)

[skilchen]

In your case1, if you start the doc comment on the template line the error goes away (i don't know why). runnableExamples will still trigger the error, but you could write something like:

when defined(case1):
  template items*[A,B](t: Table[A,B]): tuple[key: A, val: B] = ##
    ## iterates over (key, value) tuples in the table `t`.
    ##
    ## Example:
    ##
    ## .. code-block:: nim
    ##    import sequtils
    ##    let a = {10: "foo", 12: "bar"}.toTable
    ##    doAssert mapIt(a, it.key) == [10, 12]
    ##    doAssert mapIt(a, it.val)[1] == "bar"
    ##
    pairs(t)

You should come up with a better use case than this bizarre way of writing toSeq(keys(a)) and toSeq(values(a))[1]. In your PR #9217 please give an example in which an items iterator on tables would actually be useful.

[Araq]

Please, please stop writing these when defined(case1): they are so messy. What is "case1"? Does it fail? Does it work? Should it compile and doesn't or does it compile and shouldn't?

[pmetras]

The reason of the bug is that the test here considers that iterator is not used in a for loop because a doc comment adds a statement in the parsed tree, contrarily to a simple comment.

Another example of such bug where using a template is required by the context:

type
  Melon = object

template pairs*(m: Melon): tuple[key: int, val: int] =
  let iter = iterator(melon: Melon): tuple[key: int, val: int] =
    for i in a .. 10:
      yield (key: i, val: i * i)
  iter(m)

block:
  let a = 2
  let m = Melon()
  for k, v in m:
    echo "k=", k, "; v=", v

[timotheecour]

this crops up in many places, eg #14420 (comment)

when true: # D20200525T015650
  type Foo = object
    x: float

  proc fn(a: var Foo): var float =
    ## discard <- turn this into a comment (or a `discard`) and error disappears
    # result = a.x # this would work
    a.x #  Error: limited VM support for 'addr'

  template main =
    var a = Foo()
    discard fn(a)
  main() # works
  static: main()  # fails