PY-83339 Narrow X | None type to X after assert x

(cherry picked from commit a49863065bf26f615100b21f423a650cbebbcab3)

IJ-MR-178978

GitOrigin-RevId: 4bf02cab1e94a2ab1aaa70cb0b4578688cea88b9

python/testSrc/com/jetbrains/python/Py3TypeTest.java

@@ -4170,6 +4170,15 @@ public class Py3TypeTest extends PyTestCase {
       """);
   }
 
+  @TestFor(issues="PY-83339")
+  public void testAssertNarrowsOptionalAfterAssert() {
+    doTest("int", """
+      def foo(param: int | None):
+          assert param
+          expr = param
+      """);
+  }
+
   private void doTest(final String expectedType, final String text) {
     myFixture.configureByText(PythonFileType.INSTANCE, text);
     final PyExpression expr = myFixture.findElementByText("expr", PyExpression.class);