Fix #3024 (comment)

Author: kdeldycke

docs/options.md

@@ -314,15 +314,15 @@ If you want to define an alias for the second option only, then you will need to
 
 ## Flag Value
 
-To have an flag pass a value to the underlying function set `flag_value`. This automatically sets `is_flag=True`. To set a default flag, set `default=True`. Setting flag values can be used to create patterns like this:
+To have an flag pass a value to the underlying function set `flag_value`. This automatically sets `is_flag=True`. To influence the value-less behavior, force its value with `default='upper'`. Setting flag values can be used to create patterns like this:
 
 ```{eval-rst}
 .. click:example::
 
     import sys
 
     @click.command()
-    @click.option('--upper', 'transformation', flag_value='upper', default=True)
+    @click.option('--upper', 'transformation', flag_value='upper', default='upper')
     @click.option('--lower', 'transformation', flag_value='lower')
     def info(transformation):
         click.echo(getattr(sys.platform, transformation)())

src/click/core.py

@@ -2250,9 +2250,12 @@ def consume_value(
 
         If no value is found, the value is returned as :attr:`UNSET`.
         """
+        # If the value is set, it means the parser produced a value from user input.
         value = opts.get(self.name, UNSET)  # type: ignore
         source = (
-            ParameterSource.DEFAULT if value is UNSET else ParameterSource.COMMANDLINE
+            ParameterSource.COMMANDLINE
+            if value is not UNSET
+            else ParameterSource.DEFAULT
         )
 
         if value is UNSET:
@@ -3103,7 +3106,7 @@ def consume_value(
         elif (
             self.multiple
             and value is not UNSET
-            and source is ParameterSource.COMMANDLINE
+            and source not in (ParameterSource.DEFAULT, ParameterSource.DEFAULT_MAP)
             and any(v is _flag_needs_value for v in value)
         ):
             value = [self.flag_value if v is _flag_needs_value else v for v in value]

tests/test_basic.py

@@ -276,6 +276,27 @@ def cli(flag):
     assert result.output == expect
 
 
+@pytest.mark.parametrize(
+    ("default", "args", "expect"),
+    (
+        # See: https://github.com/pallets/click/issues/3024#issuecomment-3146199461
+        (True, ["--upper"], "upper"),
+        (True, ["--lower"], "lower"),
+        (True, [], "True"),
+        ("upper", [], "upper"),
+    ),
+)
+def test_flag_value(runner, default, args, expect):
+    @click.command()
+    @click.option("--upper", "transformation", flag_value="upper", default=default)
+    @click.option("--lower", "transformation", flag_value="lower")
+    def cli(transformation):
+        click.echo(repr(transformation), nl=False)
+
+    result = runner.invoke(cli, args)
+    assert result.output == repr(expect)
+
+
 def test_file_option(runner):
     @click.command()
     @click.option("--file", type=click.File("w"))