First attempt to reduce test case.

src/config.nim

@@ -1642,7 +1642,7 @@ var IDENTIFIER_PATTERN = re(r"(*U)^(?!\d)(\w+)$")
 proc isIdentifier(s: string): bool =
   s.match(IDENTIFIER_PATTERN).isSome
 
-proc parsePath(s: string): ASTNode =
+proc parsePath*(s: string): ASTNode =
 
   proc fail(loc: Location) =
     raise newConfigError(&"invalid path: {s}", loc)

tests/test3.nim

@@ -0,0 +1,38 @@
+#
+# Copyright 2016-2023 by Vinay Sajip. All Rights Reserved.
+#
+import complex
+import os
+import strutils
+import tables
+import times
+import typetraits
+import unittest
+
+import ../src/config
+
+proc dataFilePath(paths: varargs[string]): string =
+  "tests" / "resources" / paths.join("/")
+
+#
+# Config
+#
+
+proc CV(s: string): ConfigValue =
+  ConfigValue(kind: StringValue, stringValue: s)
+
+
+test "main config":
+  var p = dataFilePath("derived", "main.cfg")
+  var cfg = newConfig()
+  cfg.includePath.add(dataFilePath("base"))
+  cfg.loadFile(p)
+
+  var lcfg = cfg.getSubConfig("logging")
+  var ev = CV("bar")
+  check ev == lcfg.get("foo.bar", ev)
+  #ev.stringValue = "bozz"
+  echo "Before test"
+  discard parsePath("handlers.debug.lvl")
+  #check ev == lcfg.get("handlers.debug.lvl", ev)
+  echo "After test"