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006a0a494a
This change forces you to write ``@reparented` relationships
on an extension of a protocol, rather than on the protocol
itself.
The ProtocolConformance needs to be associated with some
GenericContext and IRGen expects it to be an ExtensionDecl.
That environment defines under what conditions the conformance
exists. We also need to define witnesses for the new parent's
requirements in an extension anyway, so it's a natural fit.
The previous workaround for this was kind of awful, as it'd
require searching all the protocol's extensions and "guess"
which extension they want to represent the conformance. While
we could try to synthesize an extension, there's two
challenges there:
1. Due to SuppressedAssociatedTypes, it's not so simple to
synthesize an unconstrained ExtensionDecl.
2. We currently rely on same-type requirements to pin the
associated types to particular witnesses of those requirements
in the extension. So it's not purely unconstrained! For example,
```
extension Seq: @reparented BorrowSeq where Iter == MyIter {}
```
The constraints that are disallowed (but not yet diagnosed)
are conditional conformance requirements, as the default
conformance for a reparenting cannot depend on those.
Thus, it's better that programmers to specify the extension.