Allow protocols with sufficient same-type constraints to be used as a regular type

[xwu (Xiaodi Wu)]

This has been discussed multiple times before. An implementation and proposal are already in progress:

Why can't protocol which inherits from PAT but constrains its associated types be used as existential?

Hi guys (@filip-sakel, @anthonylatsis), I just wanted to poke you both with regard to this feature because it came up again in my work just now, and I'll breath such a sigh of relief when this feature is eventually deployed. I myself am totally swamped with coding items on my todo list so I understand entirely that you guys may not have the time, but I just want to express the gratitude that I'll have if you do end up implementing it!