Fix an if-splitting bug.

In our implementation of COMPARE_OP, we were looping through all possible
bindings of the left and right side, then calling the comparison magic method
with all the left bindings that could not be matched definitely, and the
*entire right variable*. Just as for the left side, we should be filtering out
the right bindings that were already matched.

PiperOrigin-RevId: 343433846

pytype/tests/py3/test_splits.py

@@ -243,6 +243,15 @@ class MyIterable(Iterable[T]): ...
       def f(x: MyIterable[int]) -> Union[int, str]: ...
     """)
 
+  def test_str_none_eq(self):
+    self.Check("""
+      from typing import Optional
+      def f(x: str, y: Optional[str]) -> str:
+        if x == y:
+          return y
+        return x
+    """)
+
 
 class SplitTestPy3(test_base.TargetPython3FeatureTest):
   """Tests for if-splitting in Python 3."""

pytype/vm.py

@@ -2056,17 +2056,20 @@ def _cmp_rel(self, state, op_name, x, y):
     # A variable of the values without a special cmp_rel implementation. Needed
     # because overloaded __eq__ implementations do not necessarily return a
     # bool; see, e.g., test_overloaded in test_cmp.
-    leftover = self.program.NewVariable()
+    leftover_x = self.program.NewVariable()
+    leftover_y = self.program.NewVariable()
     for b1 in x.bindings:
       for b2 in y.bindings:
         val = compare.cmp_rel(self, getattr(slots, op_name), b1.data, b2.data)
         if val is None:
-          leftover.AddBinding(b1.data, {b1}, state.node)
+          leftover_x.AddBinding(b1.data, {b1}, state.node)
+          leftover_y.AddBinding(b2.data, {b2}, state.node)
         else:
           ret.AddBinding(self.convert.bool_values[val], {b1, b2}, state.node)
-    if leftover.bindings:
+    if leftover_x.bindings:
       op = "__%s__" % op_name.lower()
-      state, leftover_ret = self.call_binary_operator(state, op, leftover, y)
+      state, leftover_ret = self.call_binary_operator(
+          state, op, leftover_x, leftover_y)
       ret.PasteVariable(leftover_ret, state.node)
     return state, ret