Lambda that captures extra parameters cannot be connected to signal that has default argument values

Consider the lambda in this loop. It captures the extra parameter b so that setText is called on the correct widget:

for i in range(3):
    button = QCheckBox("Click me")
    button.clicked.connect(
        lambda checked, b=button: b.setText("Checked!" if checked else "Unchecked!"))

As of PySide6 6.6.0, clicking a checkbox in this example raises:

TypeError: <lambda>() missing 1 required positional argument: 'checked'

The signal QAbstractButton::clicked has one argument checked which has a default value.

The issue does not occur when connecting a lambda to a signal that has no default values for its arguments. For example, connecting a similar lambda to QCheckBox::stateChanged works fine:

button.stateChanged.connect(lambda state, b=button: b.setText(f"New state: {state}"))

Note: The attached example file produces the expected results under your competitor's implementation.