Disallow use of is_flag and multiple in options

CHANGES.rst

@@ -7,6 +7,9 @@ Unreleased
 
 -   Use verbose form of ``typing.Callable`` for ``@command`` and
     ``@group``. :issue:`2255`
+-   Show error when attempting to create an option with
+    ``multiple=True, is_flag=True``. Use ``count`` instead.
+    :issue:`2246`
 
 
 Version 8.1.2

src/click/core.py

@@ -2580,6 +2580,9 @@ def __init__(
                 if self.is_flag:
                     raise TypeError("'count' is not valid with 'is_flag'.")
 
+            if self.multiple and self.is_flag:
+                raise TypeError("'multiple' is not valid with 'is_flag', use 'count'.")
+
     def to_info_dict(self) -> t.Dict[str, t.Any]:
         info_dict = super().to_info_dict()
         info_dict.update(

tests/test_options.py

@@ -904,3 +904,21 @@ def test_type_from_flag_value():
 )
 def test_is_bool_flag_is_correctly_set(option, expected):
     assert option.is_bool_flag is expected
+
+
+@pytest.mark.parametrize(
+    ("kwargs", "message"),
+    [
+        ({"count": True, "multiple": True}, "'count' is not valid with 'multiple'."),
+        ({"count": True, "is_flag": True}, "'count' is not valid with 'is_flag'."),
+        (
+            {"multiple": True, "is_flag": True},
+            "'multiple' is not valid with 'is_flag', use 'count'.",
+        ),
+    ],
+)
+def test_invalid_flag_combinations(runner, kwargs, message):
+    with pytest.raises(TypeError) as e:
+        click.Option(["-a"], **kwargs)
+
+    assert message in str(e.value)