Python: test demonstrating the need for phi nodes

or a dataflow node playing that role, at least.

python/ql/test/experimental/dataflow/local-flow/def_use_counts.expected

@@ -0,0 +1,31 @@
+def_count
+| 4 |
+def
+| def_use_flow.py:6:5:6:5 | Essa node definition |
+| def_use_flow.py:13:11:13:11 | Essa node definition |
+| def_use_flow.py:15:9:15:9 | Essa node definition |
+| def_use_flow.py:17:7:17:7 | Essa node definition |
+implicit_use_count
+| 0 |
+implicit_use
+source_use_count
+| 3 |
+source_use
+| def_use_flow.py:24:15:24:15 | ControlFlowNode for x |
+| def_use_flow.py:26:13:26:13 | ControlFlowNode for x |
+| def_use_flow.py:28:11:28:11 | ControlFlowNode for x |
+def_use_edge_count
+| 12 |
+def_use_edge
+| def_use_flow.py:6:5:6:5 | SSA variable x | def_use_flow.py:24:15:24:15 | ControlFlowNode for x |
+| def_use_flow.py:6:5:6:5 | SSA variable x | def_use_flow.py:26:13:26:13 | ControlFlowNode for x |
+| def_use_flow.py:6:5:6:5 | SSA variable x | def_use_flow.py:28:11:28:11 | ControlFlowNode for x |
+| def_use_flow.py:13:11:13:11 | SSA variable x | def_use_flow.py:24:15:24:15 | ControlFlowNode for x |
+| def_use_flow.py:13:11:13:11 | SSA variable x | def_use_flow.py:26:13:26:13 | ControlFlowNode for x |
+| def_use_flow.py:13:11:13:11 | SSA variable x | def_use_flow.py:28:11:28:11 | ControlFlowNode for x |
+| def_use_flow.py:15:9:15:9 | SSA variable x | def_use_flow.py:24:15:24:15 | ControlFlowNode for x |
+| def_use_flow.py:15:9:15:9 | SSA variable x | def_use_flow.py:26:13:26:13 | ControlFlowNode for x |
+| def_use_flow.py:15:9:15:9 | SSA variable x | def_use_flow.py:28:11:28:11 | ControlFlowNode for x |
+| def_use_flow.py:17:7:17:7 | SSA variable x | def_use_flow.py:24:15:24:15 | ControlFlowNode for x |
+| def_use_flow.py:17:7:17:7 | SSA variable x | def_use_flow.py:26:13:26:13 | ControlFlowNode for x |
+| def_use_flow.py:17:7:17:7 | SSA variable x | def_use_flow.py:28:11:28:11 | ControlFlowNode for x |

python/ql/test/experimental/dataflow/local-flow/def_use_counts.ql

@@ -0,0 +1,47 @@
+import python
+private import semmle.python.dataflow.new.internal.DataFlowPrivate
+
+query int def_count() {
+  exists(SsaSourceVariable x | x.getName() = "x" |
+    result = count(EssaNodeDefinition def | def.getSourceVariable() = x)
+  )
+}
+
+query EssaNodeDefinition def() {
+  exists(SsaSourceVariable x | x.getName() = "x" | result.getSourceVariable() = x)
+}
+
+query int implicit_use_count() {
+  exists(SsaSourceVariable x | x.getName() = "x" | result = count(x.getAnImplicitUse()))
+}
+
+query ControlFlowNode implicit_use() {
+  exists(SsaSourceVariable x | x.getName() = "x" | result = x.getAnImplicitUse())
+}
+
+query int source_use_count() {
+  exists(SsaSourceVariable x | x.getName() = "x" | result = count(x.getASourceUse()))
+}
+
+query ControlFlowNode source_use() {
+  exists(SsaSourceVariable x | x.getName() = "x" | result = x.getASourceUse())
+}
+
+query int def_use_edge_count() {
+  exists(SsaSourceVariable x | x.getName() = "x" |
+    result =
+      count(EssaVariable v, NameNode use |
+        v.getSourceVariable() = x and
+        use = x.getAUse() and
+        LocalFlow::defToFirstUse(v, use)
+      )
+  )
+}
+
+query predicate def_use_edge(EssaVariable v, NameNode use) {
+  exists(SsaSourceVariable x | x.getName() = "x" |
+    v.getSourceVariable() = x and
+    use = x.getAUse() and
+    LocalFlow::defToFirstUse(v, use)
+  )
+}

python/ql/test/experimental/dataflow/local-flow/def_use_flow.py

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+# This test file is inspired by
+# `csharp/ql/test/library-tests/dataflow/local/UseUseExplosion.cs`
+# but with `n=3` kept small, since we do have the explosion.
+
+cond = ...
+
+# global variables are slightly special,
+# so we go into a function scope
+def scope():
+    x = 0
+
+    if(cond > 3):
+      if(cond > 2):
+        if(cond > 1):
+           pass
+        else:
+          x = 1
+      else:
+        x = 2
+    else:
+      x = 3
+
+    if(cond > 3):
+      if(cond > 2):
+        if(cond > 1):
+           pass
+        else:
+          use(x)
+      else:
+        use(x)
+    else:
+      use(x)
+
+    def use(v):
+        # this could just be `pass` but we do not want it optimized away.
+        y = v+2