fix #11897: error on given pattern in val def

[bishabosha]

fixes #11897
fixes #12629

[smarter]

I'm not familiar with this code, could you describe in the commit message what it did previously and what it does differently now?

[bishabosha]

This change will break source compatibility as now given pattern defs will shadow other givens in scope - so I imagine this should require some language import?

Edit (31/05/2021) - This change will break source compatibility as now given pattern in val defs will cause errors - so I imagine this should require some language import?

[bishabosha]

it seems val given Int = 1 has the same tree as given Int = 1, causing an error unexpectedly, so will need to go back to drawing board on that, or just drop the change.

Edit - not applicable

[smarter]

This change will break source compatibility as now given pattern defs will shadow other givens in scope - so I imagine this should require some language import?

Source-breaking change can be introduced in 3.1, but I would also prefer just not allowing givens in pattern definitions, I don't think we need another way of defining givens.

[bishabosha]

This PR has now changed to make it an error to include given patterns in a val definition - this is still source breaking change - so how should it be introduced?

[smarter]

Given that it did nothing before and was not advertised as a feature, I'm inclined to get it in without worrying about the hypothetical source compat breakage.

[bishabosha]

I have changed the implementation and adjusted the error message