Template can not be used to evaluate to iterator expression

[PhilippMDoerner]

What happened?

I'm trying to have a template evaluate to a specific iterator under varying circumstances. While the direct expression that I want (e.g. A().fieldPairs) works when entered, it can not work when you put the very same code into a template.

Minimal example:

template fieldPairsR(t: typedesc[T]) =
  T().fieldPairs

type A = object
  field1: string
  field2: string

# This compiles
for name, value in A().fieldPairs:
  echo name

# This does not compile, even though fieldPairsR(A) should be evaluating to "A().fieldPairs"
for name, value in fieldPairsR(A):
  echo name

Nim Version

Nim Compiler Version 1.7.1 [Linux: amd64] Compiled at 2022-08-04 Copyright (c) 2006-2022 by Andreas Rumpf

active boot switches: -d:release

Current Standard Output Logs

Error: attempting to call routine: 'fieldPairs'
  found iterators.fieldPairs(x: S: tuple or object, y: T: tuple or object) [iterator declared in /usr/lib/nim/system/iterators.nim(323, 10)]
  found iterators.fieldPairs(x: T: tuple or object) [iterator declared in /usr/lib/nim/system/iterators.nim(295, 10)]

Expected Standard Output Logs

field1
field2

Possible Solution

I have no idea what's broken here, this is outside of my level of expertise.

Additional Information

According to leorize this (or similar issues?) appear to be known bugs. I went over issues with the keywords "template" and "iterator" but couldn't find any that were exact matches. If there are any and I missed them, please say so.

[ringabout]

Workaround

template fieldPairsR[T](t: typedesc[T]): untyped =
  T().fieldPairs

type A = object
  field1: string
  field2: string

# This compiles
for name, value in A().fieldPairs:
  echo name

for name, value in fieldPairsR(A):
  echo name

[PhilippMDoerner]

Wait, given that that works, does that even make this a bug still?

[ringabout]

I'm not experienced in templates/macros either.

According to https://scripter.co/notes/nim/#return-type-of-templates

Thanks to @mratsim from GitHub for this reply to my question regarding untyped return type for templates:

untyped is useful to make sure the replacement is done before any semantic checking/type resolution.

If you are getting confused about whether or not to set a template’s return type or what to set it to, just set it to untyped.

If in doubt, set a template’s return type as untyped.

[ringabout]

Wait, given that that works, does that even make this a bug still?

It probably works as expected, closing.