fixes #19457 seqs are not properly updated in loop with ARC/ORC (#19922)

* test CI

* fixes #19457

* add comments

Co-authored-by: sandytypical <43030857+xflywind@users.noreply.github.com>

compiler/varpartitions.nim

@@ -767,6 +767,11 @@ proc traverse(c: var Partitions; n: PNode) =
     #     mutate(graph)
     #     connect(graph, cursorVar)
     for child in n: traverse(c, child)
+
+    if n.kind == nkWhileStmt:
+      traverse(c, n[0])
+      # variables in while condition has longer alive time than local variables 
+      # in the while loop body
   else:
     for child in n: traverse(c, child)
 
@@ -854,6 +859,11 @@ proc computeLiveRanges(c: var Partitions; n: PNode) =
     inc c.inLoop
     for child in n: computeLiveRanges(c, child)
     dec c.inLoop
+
+    if n.kind == nkWhileStmt:
+      computeLiveRanges(c, n[0])
+      # variables in while condition has longer alive time than local variables 
+      # in the while loop body
   of nkElifBranch, nkElifExpr, nkElse, nkOfBranch:
     inc c.inConditional
     for child in n: computeLiveRanges(c, child)

tests/arc/t19457.nim

@@ -0,0 +1,16 @@
+discard """
+  matrix: "--gc:refc; --gc:arc"
+"""
+
+# bug #19457
+proc gcd(x, y: seq[int]): seq[int] =
+    var
+      a = x
+      b = y
+    while b[0] > 0:
+      let c = @[a[0] mod b[0]]
+      a = b
+      b = c
+    return a
+
+doAssert gcd(@[1], @[2]) == @[1]

tests/arc/topt_refcursors.nim

@@ -5,21 +5,28 @@ discard """
 
 var
   it_cursor
-  jt_cursor
-it_cursor = root
-block :tmp:
-  while (
-    not (it_cursor == nil)):
-    echo [it_cursor.s]
-    it_cursor = it_cursor.ri
-jt_cursor = root
-block :tmp_1:
-  while (
-    not (jt_cursor == nil)):
-    var ri_1_cursor
-    ri_1_cursor = jt_cursor.ri
-    echo [jt_cursor.s]
-    jt_cursor = ri_1_cursor
+  jt
+try:
+  it_cursor = root
+  block :tmp:
+    while (
+      not (it_cursor == nil)):
+      echo [it_cursor.s]
+      it_cursor = it_cursor.ri
+  `=copy`(jt, root)
+  block :tmp_1:
+    while (
+      not (jt == nil)):
+      var ri_1
+      try:
+        `=copy`(ri_1, jt.ri)
+        echo [jt.s]
+        `=sink`(jt, ri_1)
+        wasMoved(ri_1)
+      finally:
+        `=destroy`(ri_1)
+finally:
+  `=destroy`(jt)
 -- end of expandArc ------------------------'''
 """