Bound and supertrait bounds can be empty

[bluss]

These examples all compile:

trait A : { }

struct Foo<T: > { t: T }

fn foo<T: >() { }

I expected empty bounds lists to be an error.

[Gankra]

I could see this being useful for macros?

[eddyb]

cc @petrochenkov

[petrochenkov]

It was decided this is valid syntax, see #37511 and #39158.