fix nim-lang#7187 toSeq twice; adds test case

lib/pure/collections/sequtils.nim

@@ -513,9 +513,10 @@ template toSeq*(iter: untyped): untyped =
   ##   assert odd_numbers == @[1, 3, 5, 7, 9]
 
   when compiles(iter.len):
+    evalOnce(t_D20180808T230638, iter, letAssigneable = true)
     var i = 0
-    var result = newSeq[type(iter)](iter.len)
-    for x in iter:
+    var result = newSeq[type(iter)](t_D20180808T230638.len)
+    for x in t_D20180808T230638:
       result[i] = x
       inc i
     result
@@ -1012,6 +1013,16 @@ when isMainModule:
           result = true)
     assert odd_numbers == @[1, 3, 5, 7, 9]
 
+  block:
+    # tests https://github.com/nim-lang/Nim/issues/7187
+    var counter = 0
+    proc getInput():auto =
+      counter.inc
+      @[1, 2, 3]
+    let ret = toSeq(getInput().filter(proc (x: int): bool = x < 3))
+    doAssert ret == @[1, 2]
+    doAssert counter == 1
+
   block: # foldl tests
     let
       numbers = @[5, 9, 11]
@@ -1068,13 +1079,13 @@ when isMainModule:
     doAssert openArray[int]([1,2]).mapIt(it) == @[1,2]
 
   block:
+    # tests https://github.com/nim-lang/Nim/issues/7187:
+    # make sure argument evaluated only once, analog to
     var counter = 0
     proc getInput():auto =
       counter.inc
       [1, 2]
     doAssert getInput().mapIt(it*2).mapIt(it*10) == @[20, 40]
-    # make sure argument evaluated only once, analog to
-    # https://github.com/nim-lang/Nim/issues/7187 test case
     doAssert counter == 1
 
   block: # mapIt with invalid RHS for `let` (#8566)