fixes #15361 (#15401)

* fixes #15361; better cursor inference

tests/arc/top_no_cursor2.nim

@@ -0,0 +1,53 @@
+discard """
+  output: '''true
+true
+true
+true
+true'''
+  cmd: "nim c --gc:arc $file"
+"""
+# bug #15361
+
+type
+  ErrorNodeKind = enum Branch, Leaf
+  Error = ref object
+    case kind: ErrorNodeKind
+      of Branch:
+        left: Error
+        right: Error
+      of Leaf:
+        leafError: string
+    input: string
+
+proc ret(input: string, lefterr, righterr: Error): Error =
+  result = Error(kind: Branch, left: lefterr, right: righterr, input: input)
+
+proc parser() =
+  var rerrors: Error
+  let lerrors = Error(
+    kind: Leaf,
+    leafError: "first error",
+    input: "123 ;"
+  )
+  # If you remove "block" - everything works
+  block:
+    let rresult = Error(
+      kind: Leaf,
+      leafError: "second error",
+      input: ";"
+    )
+    # this assignment is needed too
+    rerrors = rresult
+
+  # Returns Error(kind: Branch, left: lerrors, right: rerrors, input: "some val")
+  # needs to be a proc call for some reason, can't inline the result
+  var data = ret(input = "some val", lefterr = lerrors, righterr = rerrors)
+
+  echo data.left.leafError == "first error"
+  echo data.left.input == "123 ;"
+  # stacktrace shows this line
+  echo data.right.leafError == "second error"
+  echo data.right.input == ";"
+  echo data.input == "some val"
+
+parser()