`default impl SuperTrait for impl SubTrait` => trait requirement evaluation overflow

[CAD97]

I tried this code:

#![feature(specialization)]

trait Spam {}
trait SpamMore: Spam {}

default impl<T> Spam for T where T: SpamMore {}

struct A;
impl SpamMore for A {}

I expected to see this happen:

An error such as

error[[E0277]](https://doc.rust-lang.org/nightly/error-index.html#E0277): the trait bound `A: Spam` is not satisfied
 --> src/lib.rs:9:6
  |
9 | impl SpamMore for A {}
  |      ^^^^^^^^ the trait `Spam` is not implemented for `A`
  |
note: required by a bound in `SpamMore`
 --> src/lib.rs:4:17
  |
4 | trait SpamMore: Spam {}
  |                 ^^^^ required by this bound in `SpamMore`

perhaps with a hint that a default impl applies, but does not actually fulfill the trait requirement.

Instead, this happened:

Trait requirement evaluation overflowed.

error[[E0275]](https://doc.rust-lang.org/nightly/error-index.html#E0275): overflow evaluating the requirement `A: Spam`
 --> src/lib.rs:9:6
  |
9 | impl SpamMore for A {}
  |      ^^^^^^^^
  |
note: required by a bound in `SpamMore`
 --> src/lib.rs:4:17
  |
4 | trait SpamMore: Spam {}
  |                 ^^^^ required by this bound in `SpamMore`

It seems that the compiler sees the obligation A: SpamMore -> A: Spam, then incorrectly considers the obligation A: Spam -> A: SpamMore to apply the default impl... even though a) this is the root bound we're trying to solve for, and more importantly b) the implication A: SpamMore -> A: Spam doesn't even apply, since default impl doesn't satisfy the trait without a concrete impl.

An equivalent pattern using min_specialization (a default fn and not default impl) compiles cleanly:

#![feature(min_specialization)]

trait Spam {
    fn f();
}
trait SpamMore: Spam {}

impl<T> Spam for T where T: SpamMore {
    default fn f() {}
}

struct A;
impl SpamMore for A {}

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playground 1.63.0-nightly (2022-06-23 4334739)

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