Python: Address Copilot's comments

python/ql/test/library-tests/ControlFlow/evaluation-order/TimerUtils.qll

@@ -1,9 +1,10 @@
 /**
  * Utility library for identifying timer annotations in evaluation-order tests.
  *
- * Identifies `expr @ t[n]` (matmul), `t(expr, n)` (call), and
+ * Identifies `expr @ t[n]` (matmul) and `t(expr, n)` (call) patterns,
- * `expr @ t.dead[n]` (dead-code) patterns, extracts timestamp values,
+ * including `dead(n)` and `never` markers within subscripts, extracts
- * and provides predicates for traversing consecutive annotated CFG nodes.
+ * timestamp values, and provides predicates for traversing consecutive
+ * annotated CFG nodes.
  */
 
 import python

python/ql/test/library-tests/ControlFlow/evaluation-order/test_basic.py

@@ -5,7 +5,7 @@ are evaluated in the expected order (typically left to right for
 operands of binary operators, elements of collection literals, etc.)
 
 Every evaluated expression has a timestamp annotation, except the
-timer mechanism itself (t[n], t.dead[n]).
+timer mechanism itself (t[n], t[dead(n)], t[never]).
 """
 
 from timer import test, never

python/ql/test/library-tests/ControlFlow/evaluation-order/timer.py

@@ -26,6 +26,7 @@ Run a test file directly to self-validate: python test_file.py
 """
 
 import atexit
+import os
 import sys
 
 _results = []
@@ -53,6 +54,10 @@ class _Check:
                 self._dead.add(e.timestamp)
             elif isinstance(e, _NeverSentinel):
                 self._never = True
+            else:
+                raise TypeError(
+                    f"Unknown element in timer subscript: {e!r} (type {type(e).__name__})"
+                )
 
     def __rmatmul__(self, value):
         ts = self._timer._tick()
@@ -183,7 +188,7 @@ def _report():
     print("---")
     print(f"{passed}/{total} tests passed")
     if passed < total:
-        sys.exit(1)
+        os._exit(1)
 
 
 atexit.register(_report)