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a171994070
consider item bounds for non-yet-defined opaque types Based on rust-lang/rust#140405. fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/182 fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/196 fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/205 there's some jank here, see https://github.com/rust-lang/trait-system-refactor-initiative/issues/229 ## Design If the self type is an inference variable which has been sub-unified with am opaque type, we need to incompletely guide inference to avoid breakage. In this case, we - look at the item bounds of all sub-unified opaque types, and - blanket impls which do not constrain the self type Even if there are applicable candidates, we always force their certainty to be `Maybe`, so they will always have to be reproven once we've constrained the inference variable. This is a bit iffy, see the added tests. r? `@BoxyUwU`
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