only use recorded liveness version info during slicing

nbsafety/safety.py

@@ -509,14 +509,8 @@ def _get_cell_dependencies(
         dependencies.add(cell_num)
 
         # Retrieve cell numbers for the dependent symbols
-        cell = self.cell_content_by_counter[cell_num]
-        live_symbols = self._check_cell_and_resolve_symbols(cell)['live']
-        dep_cell_nums = set(
-            dep_symbol.defined_cell_num for dep_symbol in live_symbols
-        )
         # Add dynamic and static dependencies
-        dep_cell_nums = dep_cell_nums | cell_num_to_dynamic_deps[cell_num]
-        dep_cell_nums = dep_cell_nums | cell_num_to_static_deps[cell_num]
+        dep_cell_nums = cell_num_to_dynamic_deps[cell_num] | cell_num_to_static_deps[cell_num]
         logger.info("dynamic cell deps for %d: %s", cell_num,
                     cell_num_to_dynamic_deps[cell_num])
         logger.info("static cell deps for %d: %s", cell_num,

test/test_dynamic_slicing.py

@@ -83,7 +83,6 @@ def test_imports():
     assert deps == {1, 2, 3}, 'got %s' % deps
 
 
-@skipif_known_failing
 def test_handle_stale():
     run_cell('a = 1')
     run_cell('b = 2 * a')
@@ -120,3 +119,22 @@ def test_version_used_when_live():
     run_cell('logging.info(x + y)')
     deps = set(nbs().get_cell_dependencies(4).keys())
     assert deps == {1, 2, 3, 4}, 'got %s' % deps
+
+
+def test_no_definitely_spurious_cells():
+    run_cell('x = 0')
+    run_cell("""
+if True:
+    y = 7
+else:
+    # even though this branch is not taken,
+    # liveness-based usage should detect the
+    # version of `x` used at the time it was
+    # live, meaning cell 1 should get included
+    # in the slice
+    logging.info(x)
+""")
+    run_cell('x = 5')
+    run_cell('logging.info(y)')
+    deps = set(nbs().get_cell_dependencies(4).keys())
+    assert deps == {1, 2, 4}, 'got %s' % deps