fixes #18326 (#18335)

* fixes #18326

* make tests green again

tests/isolate/tisolate.nim

@@ -1,12 +1,17 @@
 discard """
   errormsg: "expression cannot be isolated: select(a, b)"
-  line: 34
+  line: 39
 """
 
 import std / isolation
 
 import json, streams
 
+proc myParseJson(s: Stream; filename: string): JsonNode =
+  {.cast(noSideEffect).}:
+    result = parseJson(s, filename)
+
+
 proc f(): seq[int] =
   @[1, 2, 3]
 
@@ -28,7 +33,7 @@ proc main =
   discard isolate select(Node(x: "a"), nil)
   discard isolate select(Node(x: "a"), Node(x: "b"))
 
-  discard isolate parseJson(newFileStream("my.json"), "my.json")
+  discard isolate myParseJson(newFileStream("my.json"), "my.json")
 
   var a, b: Node
   discard isolate select(a, b)