Python: test dead bindings under no-raise CFG abstraction

Adds 'dead_under_no_raise.py' to the bindings test suite, capturing the
three CPython patterns where bindings legitimately have no CFG node
because the surrounding code is unreachable under the 'no expressions
raise' abstraction:

  1. Statements after a 'try: return X; except: pass' block.
  2. The 'else:' clause of a try whose body always raises.
  3. Cache-lookup pattern 'try: return cache[k]; except: pass' followed
     by computation and store.

These bindings intentionally carry no 'cfgdefines=' annotations. If
raise modelling is later added to the CFG, the BindingsTest will surface
the new CFG nodes as unexpected results and this file will need to be
revisited.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: yoff

python/ql/test/library-tests/ControlFlow/bindings/dead_under_no_raise.py

@@ -0,0 +1,52 @@
+# Dead bindings under the "no expressions raise" CFG abstraction.
+#
+# The new CFG does not currently model raise edges from arbitrary
+# expressions. As a consequence, code that is only reachable through
+# exception flow is (correctly) classified as dead and has no CFG node.
+# Variable bindings in dead code do not need CFG nodes - SSA / dataflow
+# over dead code is moot.
+#
+# These tests act as a regression guard: the bindings below intentionally
+# have no `cfgdefines=` annotations. If raise modelling is later added,
+# the BindingsTest infrastructure will surface the new CFG nodes as
+# unexpected results, and this file will need to be revisited.
+
+
+def f(obj):  # $ cfgdefines=f cfgdefines=obj
+    try:
+        return len(obj)
+    except TypeError:
+        pass
+
+    # The first try's body always returns; its except handler does not
+    # raise or otherwise transfer control, so under "no expressions
+    # raise" the only paths out of the try-statement are dead. Everything
+    # below is unreachable.
+    try:
+        hint = type(obj).__length_hint__
+    except AttributeError:
+        return None
+    return hint
+
+
+def g():  # $ cfgdefines=g
+    try:
+        raise Exception("inner")
+    except:
+        raise Exception("outer")
+    else:
+        # Unreachable: the inner try body always raises, so the `else:`
+        # clause never runs.
+        hit_inner_else = True
+
+
+def h(cache, key):  # $ cfgdefines=h cfgdefines=cache cfgdefines=key
+    try:
+        return cache[key]
+    except KeyError:
+        pass
+
+    # Same pattern as `f`: dead under "no expressions raise".
+    value = compute(key)
+    cache[key] = value
+    return value