refactor illegal iterator assignment detection (#12212)

* refactor illegal iterator assignment detection

* delete crappy test

tests/ccgbugs/t5701.nim

@@ -1,27 +0,0 @@
-discard """
-  output: '''(1, 1)
-(2, 2)
-(3, 3)
-@[1, 2, 3, 4]
-'''
-"""
-
-iterator zip[T1, T2](a: openarray[T1], b: openarray[T2]): iterator() {.inline.} =
-  let len = min(a.len, b.len)
-  for i in 0..<len:
-    echo (a[i], b[i])
-
-proc foo(args: varargs[int]) =
-  for i in zip(args,args):
-    discard
-
-foo(1,2,3)
-
-# 10999
-
-proc varargsToSeq(vals: varargs[int32]): seq[int32] =
-  result = newSeqOfCap[int32](vals.len)
-  for v in vals:
-    result.add v
-
-echo varargsToSeq(1, 2, 3, 4)

tests/ccgbugs/tccgissues.nim

@@ -0,0 +1,14 @@
+discard """
+  output: '''
+@[1, 2, 3, 4]
+'''
+"""
+
+# issue #10999
+
+proc varargsToSeq(vals: varargs[int32]): seq[int32] =
+  result = newSeqOfCap[int32](vals.len)
+  for v in vals:
+    result.add v
+
+echo varargsToSeq(1, 2, 3, 4)

tests/errmsgs/ttypeAllowed.nim

@@ -0,0 +1,28 @@
+discard """
+cmd: "nim check $file"
+errmsg: ""
+nimout: '''
+ttypeAllowed.nim(13, 5) Error: invalid type: 'iterator (a: int, b: int, step: Positive): int{.inline, noSideEffect, gcsafe, locks: 0.}' for let
+ttypeAllowed.nim(17, 7) Error: invalid type for const: iterator (a: int, b: int, step: Positive): int{.inline, noSideEffect, gcsafe, locks: 0.}
+ttypeAllowed.nim(21, 5) Error: invalid type: 'iterator (a: int, b: int, step: Positive): int{.inline, noSideEffect, gcsafe, locks: 0.}' for var
+ttypeAllowed.nim(26, 10) Error: invalid type: 'iterator (a: int, b: int, step: Positive): int{.inline, noSideEffect, gcsafe, locks: 0.}' for result
+'''
+"""
+
+
+let f1 = case true
+  of true:  countup[int]
+  of false: countdown[int]
+
+const f2 = case true
+  of true:  countup[int]
+  of false: countdown[int]
+
+var f3 = case true
+  of true:  countup[int]
+  of false: countdown[int]
+
+proc foobar(): auto =
+  result = case true
+    of true:  countup[int]
+    of false: countdown[int]

tests/iter/titerassignerr.nim

@@ -1,8 +0,0 @@
-discard """
-  errormsg: "inline iterators are not first-class / cannot be assigned to variables"
-  line: 8
-"""
-
-iterator foo: int =
-  yield 2
-let x = foo