allow multiple=True with flags

co-authored-by: Amethyst Reese <amethyst@n7.gg>

CHANGES.rst

@@ -22,6 +22,8 @@ Unreleased
     ``standalone_mode`` is disabled. :issue:`2380`
 -   ``@group.command`` does not fail if the group was created with a custom
     ``command_class``. :issue:`2416`
+-   ``multiple=True`` is allowed for flag options again and does not require
+    setting ``default=()``. :issue:`2246, 2292, 2295`
 
 
 Version 8.1.3

src/click/core.py

@@ -2570,8 +2570,13 @@ def __init__(
             # flag if flag_value is set.
             self._flag_needs_value = flag_value is not None
 
+        self.default: t.Union[t.Any, t.Callable[[], t.Any]]
+
         if is_flag and default_is_missing and not self.required:
-            self.default: t.Union[t.Any, t.Callable[[], t.Any]] = False
+            if multiple:
+                self.default = ()
+            else:
+                self.default = False
 
         if flag_value is None:
             flag_value = not self.default
@@ -2622,9 +2627,6 @@ 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_defaults.py

@@ -38,3 +38,24 @@ def cli(arg):
     result = runner.invoke(cli, [])
     assert not result.exception
     assert result.output.splitlines() == ["<1|2>", "<3|4>"]
+
+
+def test_multiple_flag_default(runner):
+    """Default default for flags when multiple=True should be empty tuple."""
+
+    @click.command
+    # flag due to secondary token
+    @click.option("-y/-n", multiple=True)
+    # flag due to is_flag
+    @click.option("-f", is_flag=True, multiple=True)
+    # flag due to flag_value
+    @click.option("-v", "v", flag_value=1, multiple=True)
+    @click.option("-q", "v", flag_value=-1, multiple=True)
+    def cli(y, f, v):
+        return y, f, v
+
+    result = runner.invoke(cli, standalone_mode=False)
+    assert result.return_value == ((), (), ())
+
+    result = runner.invoke(cli, ["-y", "-n", "-f", "-v", "-q"], standalone_mode=False)
+    assert result.return_value == ((True, False), (True,), (1, -1))

tests/test_options.py

@@ -911,10 +911,6 @@ def test_is_bool_flag_is_correctly_set(option, expected):
     [
         ({"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):