fix #9778 (pairs iterator calling a helper proc with tuple return type will cut the iterator yield into half) (#18767)

* test someSymFromImportTable
* Update compiler/lookups.nim
* test nkTupleConstr
* use isConstExpr
* add tests
* add comments and todo
* use todo

tests/stdlib/tyield.nim

@@ -0,0 +1,256 @@
+discard """
+  targets: "c cpp js"
+"""
+
+import std/[sugar, algorithm]
+
+block:
+  var x = @[(6.0, 6, '6'),
+            (5.0, 5, '5'),
+            (4.0, 4, '4'),
+            (3.0, 3, '3'),
+            (2.0, 2, '2'),
+            (1.0, 1, '1')]
+
+  let y = x.reversed
+
+  block:
+    let res = collect:
+      for (f, i, c) in x:
+        (f, i, c)
+
+    doAssert res == x
+
+  iterator popAscending[T](q: var seq[T]): T =
+    while q.len > 0: yield q.pop
+
+  block:
+    var res = collect:
+      for f, i, c in popAscending(x):
+        (f, i, c)
+
+    doAssert res == y
+
+    let z = reversed(res)
+    let res2 = collect:
+      for (f, i, c) in popAscending(res):
+        (f, i, c)
+
+    doAssert res2 == z
+
+
+block:
+  var visits = 0
+  block:
+    proc bar(): (int, int) =
+      inc visits
+      (visits, visits)
+
+    iterator foo(): (int, int) =
+      yield bar()
+
+    for a, b in foo():
+      doAssert a == b
+
+    doAssert visits == 1
+
+  block:
+    proc iterAux(a: seq[int], i: var int): (int, string) =
+      result = (a[i], $a[i])
+      inc i
+
+    iterator pairs(a: seq[int]): (int, string) =
+      var i = 0
+      while i < a.len:
+        yield iterAux(a, i)
+
+    var x = newSeq[int](10)
+    for i in 0 ..< x.len:
+      x[i] = i
+
+    let res = collect:
+      for k, v in x:
+        (k, v)
+
+    let expected = collect:
+      for i in 0 ..< x.len:
+        (i, $i)
+
+    doAssert res == expected
+
+  block:
+    proc bar(): (int, int, int) =
+      inc visits
+      (visits, visits, visits)
+
+    iterator foo(): (int, int, int) =
+      yield bar()
+
+    for a, b, c in foo():
+      doAssert a == b
+
+    doAssert visits == 2
+
+
+  block:
+
+    proc bar(): int =
+      inc visits
+      visits
+
+    proc car(): int =
+      inc visits
+      visits
+
+    iterator foo(): (int, int) =
+      yield (bar(), car())
+      yield (bar(), car())
+
+    for a, b in foo():
+      doAssert b == a + 1
+
+    doAssert visits == 6
+
+
+  block:
+    proc bar(): (int, int) =
+      inc visits
+      (visits, visits)
+
+    proc t2(): int = 99
+
+    iterator foo(): (int, int) =
+      yield (12, t2())
+      yield bar()
+
+    let res = collect:
+      for (a, b) in foo():
+        (a, b)
+
+    doAssert res == @[(12, 99), (7, 7)]
+    doAssert visits == 7
+
+  block:
+    proc bar(): (int, int) =
+      inc visits
+      (visits, visits)
+
+    proc t2(): int = 99
+
+    iterator foo(): (int, int) =
+      yield ((12, t2()))
+      yield (bar())
+
+    let res = collect:
+      for (a, b) in foo():
+        (a, b)
+
+    doAssert res == @[(12, 99), (8, 8)]
+    doAssert visits == 8
+
+  block:
+    proc bar(): (int, int) =
+      inc visits
+      (visits, visits)
+
+    proc t1(): int = 99
+    proc t2(): int = 99
+
+    iterator foo(): (int, int) =
+      yield (t1(), t2())
+      yield bar()
+
+    let res = collect:
+      for a, b in foo():
+        (a, b)
+
+    doAssert res == @[(99, 99), (9, 9)]
+    doAssert visits == 9
+
+
+  block:
+    proc bar(): ((int, int), string) =
+      inc visits
+      ((visits, visits), $visits)
+
+    proc t2(): int = 99
+
+    iterator foo(): ((int, int), string) =
+      yield ((1, 2), $t2())
+      yield bar()
+
+    let res = collect:
+      for a, b in foo():
+        (a, b)
+
+    doAssert res == @[((1, 2), "99"), ((10, 10), "10")]
+    doAssert visits == 10
+
+
+  block:
+    proc bar(): (int, int) =
+      inc visits
+      (visits, visits)
+
+    iterator foo(): (int, int) =
+      yield (for i in 0 ..< 10: discard bar(); bar())
+      yield (bar())
+
+    let res = collect:
+      for (a, b) in foo():
+        (a, b)
+
+    doAssert res == @[(21, 21), (22, 22)]
+
+  block:
+    proc bar(): (int, int) =
+      inc visits
+      (visits, visits)
+
+    proc t2(): int = 99
+
+    iterator foo(): (int, int) =
+      yield if true: bar() else: (t2(), t2())
+      yield (bar())
+
+    let res = collect:
+      for a, b in foo():
+        (a, b)
+
+    doAssert res == @[(23, 23), (24, 24)]
+
+
+block:
+  iterator foo(): (int, int, int) =
+    var time = 777
+    yield (1, time, 3)
+
+  let res = collect:
+    for a, b, c in foo():
+      (a, b, c)
+
+  doAssert res == @[(1, 777, 3)]
+
+block:
+  iterator foo(): (int, int, int) =
+    var time = 777
+    yield (1, time, 3)
+
+  let res = collect:
+    for t in foo():
+      (t[0], t[1], t[2])
+
+  doAssert res == @[(1, 777, 3)]
+
+
+block:
+  proc bar(): (int, int, int) =
+    (1, 2, 3)
+  iterator foo(): (int, int, int) =
+    yield bar()
+
+  let res = collect:
+    for a, b, c in foo():
+      (a, b, c)
+
+  doAssert res == @[(1, 2, 3)]