Disallow use of is_flag and multiple in options

src/click/core.py

@@ -2580,6 +2580,10 @@ def __init__(
                 if self.is_flag:
                     raise TypeError("'count' is not valid with 'is_flag'.")
 
+            if self.multiple:
+                if self.is_flag:
+                    raise TypeError("'multiple' is not valid with 'is_flag'.")
+
     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 +904,31 @@ def test_type_from_flag_value():
 )
 def test_is_bool_flag_is_correctly_set(option, expected):
     assert option.is_bool_flag is expected
+
+
+def test_invalid_flag_combinations(runner):
+    try:
+
+        @click.command()
+        @click.option("--foo", is_flag=True, count=True)
+        def cli1(foo):
+            pass
+
+    except TypeError as e:
+        assert "'count' is not valid with 'is_flag'" in str(e)
+
+    else:
+        raise AssertionError("didn't raise TypeError for use of count and is_flag")
+
+    try:
+
+        @click.command()
+        @click.option("--foo", is_flag=True, multiple=True)
+        def cli2(foo):
+            pass
+
+    except TypeError as e:
+        assert "'multiple' is not valid with 'is_flag'" in str(e)
+
+    else:
+        raise AssertionError("didn't raise TypeError for use of multiple and is_flag")