caac670a16
Generalize trait object generic param check to aliases. The current algorithm only checks that `Self` does not appear in defaults for traits. This is not sufficient for trait aliases. This PR moves the check to trait object elaboration, which sees through trait aliases. Fixes https://github.com/rust-lang/rust/issues/82927. Fixes https://github.com/rust-lang/rust/issues/84789.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.