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Document subtle implied bounds issue in RPITIT inference (#1807)

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Michael Goulet 2023-10-14 14:53:57 +01:00 committed by GitHub
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src/return-position-impl-trait-in-trait.md
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@ -298,6 +298,24 @@ types in the impl, since this mapping describes the type that should
come after the `=` in `type Assoc = ...` for each RPITIT.
</details>
##### Implied bounds in RPITIT hidden type inference
Since `collect_return_position_impl_trait_in_trait_tys` does fulfillment and
region resolution, we must provide it `assumed_wf_types` so that we can prove
region obligations with the same expected implied bounds as
`compare_method_predicate_entailment` does.
Since the return type of a method is understood to be one of the assumed WF
types, and we eagerly fold the return type with inference variables to do
opaque type inference, after opaque type inference, the return type will
resolve to contain the hidden types of the RPITITs. this would mean that the
hidden types of the RPITITs would be assumed to be well-formed without having
independently proven that they are. This resulted in a
[subtle unsoundness bug](https://github.com/rust-lang/rust/pull/116072). In
order to prevent this cyclic reasoning, we instead replace the hidden types of
the RPITITs in the return type of the method with *placeholders*, which lead
to no implied well-formedness bounds.
#### Default trait body
Type-checking a default trait body, like: