@YannickJadoul, thanks a lot! It looks really good. I will definitely merge this PR but I need a little bit of time to get familiar with the new code and understand how it solves the limitations.

@TvoroG Great! Thank you :-)
Thanks as well for making this library in the first place, by the way.

If you have any questions, do let me know.

I can give a quick, intuitive summary of the changes if that helps:

• Instead of calling fm.getfixturedefs, I call fm.getfixtureclosure and apply the parametrization for all of these, in a loop. This fixes the case where lazily-used fixtures have parametrized subfixtures.
• The one thing necessary for this is to have the "parent node" from where this fixture is called. On later versions, we can access metafunc.definition.parent. Before pytest 3.10.0, it's really hard to get hold of this (which is why you wrote get_nodeid before, I'm guessing?), but we can intercept that node from the pytest_pycollect_makeitem hook, since during this hook, pytest_generate_tests will be called.
• Finally, this other issue, of subfixtures parametrized with lazy fixtures, is resolved by an implementation of the pytest_fixture_setup hook, where we resolve request.param if it is a lazy fixture placeholder.

I am now also thinking that I will make the last commit into a different PR, since it has a different purpose. This should make it easier to review, as well :-)