Fixes flag default handling for boolean options

Ensures boolean flag options correctly determine their default values when not explicitly set.
Adds tests to validate proper behavior for various flag configurations, improving reliability.

Fixes #2897

Author: john0isaac

CHANGES.rst

@@ -3,6 +3,7 @@
 Version 8.2.1
 -------------
 
+- Fixes flag default handling for boolean options. :issue:`2897` :pr:`2908`
 
 Version 8.2.0
 -------------

src/click/core.py

@@ -2618,6 +2618,12 @@ def __init__(
             # default.
             self.type = types.convert_type(None, flag_value)
 
+        if is_flag and isinstance(self.type, types.BoolParamType):
+            # If the type is a boolean, we need to set the flag_value
+            # to True or False depending on the default.
+            if flag_value is None:
+                flag_value = not self.default
+
         self.is_flag: bool = is_flag
         self.is_bool_flag: bool = is_flag and isinstance(self.type, types.BoolParamType)
         self.flag_value: t.Any = flag_value

tests/test_defaults.py

@@ -84,3 +84,43 @@ def foo(name):
 
     result = runner.invoke(cli, ["foo", "--help"], default_map={"foo": {"name": True}})
     assert "default: name" in result.output
+
+
+def test_flag_default_bool(runner):
+    """test flag with default bool"""
+
+    @click.command()
+    @click.option("--foo", is_flag=True, default=False, type=click.BOOL)
+    @click.option("--bar", is_flag=True, default=False, type=bool)
+    @click.option("--baz", is_flag=True, default=True, type=click.BOOL)
+    @click.option("--qux", is_flag=True, default=True, type=bool)
+    def cli(foo, bar, baz, qux):
+        assert isinstance(foo, bool)
+        assert isinstance(bar, bool)
+        assert isinstance(baz, bool)
+        assert isinstance(qux, bool)
+        click.echo(f"foo: {foo}, bar: {bar}, baz: {baz}, qux: {qux}")
+
+    result = runner.invoke(cli, [])
+    assert not result.exception
+    assert "foo: False, bar: False, baz: True, qux: True" in result.output
+
+    result = runner.invoke(cli, ["--foo"])
+    assert not result.exception
+    assert "foo: True, bar: False, baz: True, qux: True" in result.output
+
+    result = runner.invoke(cli, ["--bar"])
+    assert not result.exception
+    assert "foo: False, bar: True, baz: True, qux: True" in result.output
+
+    result = runner.invoke(cli, ["--baz"])
+    assert not result.exception
+    assert "foo: False, bar: False, baz: False, qux: True" in result.output
+
+    result = runner.invoke(cli, ["--qux"])
+    assert not result.exception
+    assert "foo: False, bar: False, baz: True, qux: False" in result.output
+
+    result = runner.invoke(cli, ["--foo", "--bar", "--baz", "--qux"])
+    assert not result.exception
+    assert "foo: True, bar: True, baz: False, qux: False" in result.output