Warn on test arguments with default values?

As someone pointed out in my pytest training, pytest currently silently accepts:

def test_fixt(fixt=42):
    ...

with fixt obviously being 42 in the test function no matter if such a fixture was ever defined or not.

There doesn't really seem any valid reason to do this kind of thing, yet some ideas float around how default argument like this could be used in the future with either test functions (e.g. #3834) or with fixture functions (e.g. #8109).

To avoid accidental usage and possibly allow for different usage in the future, should we maybe add a warning if default argument values are used like this?

The issue names no files, tests, or entry points. First determine how pytest handles test-function defaults during collection and how warnings are emitted, then clarify the intended behavior in light of the possible future uses mentioned. Done means an agreed warning policy with focused coverage for default-valued test arguments.