SE-0352: Implicitly Opened Existentials

[Joe_Groff]

@Douglas_Gregor

The "upper bound" here is currently defined to be any Collection , because the language can't currently express anything tighter than that. If existentials grow the ability to handle constraints (e.g., the constrained existentials from this pitch covers most of the cases, although not your specific one), the upper bound could be tightened.

Would tightening the upper bound become a potential source break if we attempt it in the future?