Result of in operation is always bool (PY-31956)

python/src/com/jetbrains/python/psi/impl/PyBinaryExpressionImpl.java

@@ -136,6 +136,9 @@ public class PyBinaryExpressionImpl extends PyElementImpl implements PyBinaryExp
       }
       return PyUnionType.union(leftType, rightType);
     }
+
+    if (PyNames.CONTAINS.equals(getReferencedName())) return PyBuiltinCache.getInstance(this).getBoolType();
+
     final List<PyCallExpression.PyArgumentsMapping> results =
       PyCallExpressionHelper.mapArguments(this, PyResolveContext.noImplicits().withTypeEvalContext(context));
     if (!results.isEmpty()) {

python/testSrc/com/jetbrains/python/PyTypeTest.java

@@ -3305,6 +3305,16 @@ public class PyTypeTest extends PyTestCase {
            "   expr = a");
   }
 
+  // PY-31956
+  public void testInAndNotBoolContains() {
+    doTest("bool",
+                   "class MyClass:\n" +
+                   "    def __contains__(self):\n" +
+                   "        return 42\n" +
+                   "\n" +
+                   "expr = 1 in MyClass()");
+  }
+
   // PY-32533
   public void testSuperWithAnotherType() {
     runWithLanguageLevel(