There are a few cases where the two behave differently. @slavapestov might remember better than me.

Comment by Gregory Higley (JIRA)

Well, right now extension Z & P is completely disallowed. It produces a compiler error. I propose to repurpose it as syntactic sugar for extension P where Self: Z. It seems intuitively correct.

Yes, and it isn't. There are a few cases where the two behave differently. More specifically, there's a distinction between

extension P where Self: Z

and

extension Z where Self: P

but I can't remember when it comes up. It's pretty rare.

There could be a fix-it, at least, but only if it's obvious what you meant to extend.

Comment by Gregory Higley (JIRA)

I thought about the distinction you mentioned, but I can't think of a case where it would occur. If that's true, it'd be a real shame. If there is a distinction, the existing syntax is probably best, because it's clearer, though one path forward might be to eliminate the distinction and then add my tasty syntactic sugar. 🙂