Add defaults to the generic parameters of BaseExceptionGroup and ExceptionGroup (#12258)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

stdlib/@tests/test_cases/builtins/check_exception_group-py311.py

@@ -321,3 +321,34 @@ if sys.version_info >= (3, 11):
     # Note, that `Self` type is not preserved in runtime.
     assert_type(cg1.derive([ValueError()]), ExceptionGroup[ValueError])
     assert_type(cg1.derive([KeyboardInterrupt()]), BaseExceptionGroup[KeyboardInterrupt])
+
+    # Additional tests
+    # ==============================
+
+    def test_exception_group_default_type() -> None:
+        try:
+            ex: ExceptionGroup = ExceptionGroup("", [ValueError("a"), ValueError("b")])
+            raise ex
+        except ExceptionGroup as e:
+            assert all(isinstance(exc, ValueError) for exc in e.exceptions)
+
+    def test_exception_group_with_specific_type() -> None:
+        try:
+            ex: ExceptionGroup[ValueError] = ExceptionGroup("", [ValueError("a"), ValueError("b")])
+            raise ex
+        except ExceptionGroup as e:
+            assert all(isinstance(exc, ValueError) for exc in e.exceptions)
+
+    def test_base_exception_group_default_type() -> None:
+        try:
+            ex: BaseExceptionGroup = BaseExceptionGroup("", [SystemExit("a"), SystemExit("b")])
+            raise ex
+        except BaseExceptionGroup as e:
+            assert all(isinstance(exc, SystemExit) for exc in e.exceptions)
+
+    def test_base_exception_group_with_specific_type() -> None:
+        try:
+            ex: BaseExceptionGroup[SystemExit] = BaseExceptionGroup("", [SystemExit("a"), SystemExit("b")])
+            raise ex
+        except BaseExceptionGroup as e:
+            assert all(isinstance(exc, SystemExit) for exc in e.exceptions)

stdlib/builtins.pyi

@@ -2006,9 +2006,9 @@ if sys.version_info >= (3, 10):
     class EncodingWarning(Warning): ...
 
 if sys.version_info >= (3, 11):
-    _BaseExceptionT_co = TypeVar("_BaseExceptionT_co", bound=BaseException, covariant=True)
+    _BaseExceptionT_co = TypeVar("_BaseExceptionT_co", bound=BaseException, covariant=True, default=BaseException)
     _BaseExceptionT = TypeVar("_BaseExceptionT", bound=BaseException)
-    _ExceptionT_co = TypeVar("_ExceptionT_co", bound=Exception, covariant=True)
+    _ExceptionT_co = TypeVar("_ExceptionT_co", bound=Exception, covariant=True, default=Exception)
     _ExceptionT = TypeVar("_ExceptionT", bound=Exception)
 
     # See `check_exception_group.py` for use-cases and comments.