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Default sizedness bounds were not being added to `explicit_super_predicates_of` and `explicit_implied_predicates_of` which meant that a trait bound added to a associated type projection would be missing the implied predicate of the default sizedness supertrait of that trait. An unexpected consequence of this change was that the check for multiple principals was now finding an additional `MetaSized` principal when eagerly expanding trait aliases. Instead of special-casing trait aliases as different from traits and not adding a `MetaSized` supertrait to trait aliases, filter out `MetaSized` when lowering `dyn Trait`.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.