Never type fallback

[oli-obk]

Right now

trait T {}
impl T for () {}

fn should_ret_unit() -> impl T {
    panic!()
}

compiles successfully, but

trait T {}
impl T for () {}

fn rec() -> impl T {
    rec()
}

does not, even though

trait T {}
impl T for () {}

fn rec() -> ! {
    rec()
}

does with the never_type feature gate.

I think we should "just make it work" (it's actually less special casing in the compiler to make it work than to make it error).

Similarly, with type alias impl trait, when we have

trait T {}
impl T for i32 {}

type U = impl T;

fn foo() -> U {
    panic!()
}
fn bar() -> U {
    42
}

it should just compile, as foo never produces any problematic value.