unconstrained RPIT cause ambiguity errors instead of falling back to ()

[lcnr]

• affected tests
  • tests/ui/impl-trait/recursive-impl-trait-type-direct.rs
  • tests/ui/impl-trait/issue-103599.rs

trait T {}

fn wrap(x: impl T) -> impl T {
    //~^ WARN function cannot return without recursing
    wrap(wrap(x))
}

fn main() {}

[compiler-errors]

The root cause of this is probably a combination of
(1.) we can rely on opaques's item bounds holding in the defining scope even if we don't know the hidden type.
(2.) an opaque can be defined to its own opaque type, which we special-case in writeback and ultimately ignore.