for x in myIter(1)(): ... gives infinite loop; inconsistent with let it = iterGen(1); for x in it(): ...

[timotheecour]

originally reported here: https://github.com/nim-lang/Nim/issues/7047#issuecomment-390604711

proc iterGen(someEvalOnceArg: int): (iterator (): int {.closure.}) =
  result = iterator (): int =
    yield 1
    yield 2
    yield 3

# this works fine
let it = iterGen(42)
for x in it():
  echo x

# this is an infinite loop
for x in iterGen(42)():
  echo x

proposal

• make for x in expr: ... cache the value of expr to avoid this (maybe using something like sequtils.evalOnceAs)

scratch below

• maybe for could be actually implemented in system.nim, now that we have forLoopMacros; maybe this would help in solving this (not sure)

[bluenote10]

This has also been proposed here #2563