Lifting the "Self or associated type" constraint on existentials

[gwendal.roue (Gwendal Roué)]

@dabrahams

If what @xwu says is true, that most people run into this wall in places where even a generalized existential would be a poor fit for the use case, then the wall—as frustrating as it might be—is actually performing a valuable service.

I'm keen to agree with @dabrahams here. The way I understand it, shifting the error can be worse than the status quo because the amount of refactoring that has to be performed in order to remove the error can be dramatically bigger.