In Swift 5.10 if you wrote `extension Foo {}` for some protocol Foo,
the extension would always re-use the generic signature of Foo, which
is <Self where Self: Foo>. In Swift 6 this no longer works because Foo
might be ~Copyable, in which case `extension Foo {}` adds default
requirements, so we changed GenericSignatureRequest to just always
build a new signature if we're given an extension.
However, to avoid a request cycle with a code example that really should
have never worked at all, I'm re-introducing the hack for re-using the
signature.
Fixes rdar://problem/129540617.
We have two "levels" of name lookup, and the more primitive level is
used by name lookup itself to avoid certain cycles. For example,
extension binding, resolution of inheritance clauses, etc.
One interesting case is that a protocol extension can impose additional
requiremnts on `Self`, and members of the right-hand side type are
visible to unqualified lookup.
The right-hand side of a `Self` requirement in this case is always a
protocol type or class type canonically, but it might be written to
refer to a protocol type alias.
Before some changes for noncopyable generics, the primitive name
lookup mechanism, implemented in directReferencesForTypeRepr() and
such, would check if the TypeRepr had already been resolved by
resolveType(). If so, it would immediately return the decl.
This masked an issue, where the right-hand side of a `Self` requirement
was resolved in the parent DeclContext. A more subtle rule is needed;
for a protocol extension, we must resolve the right-hand side in the
protocol, but disregard the protocol extension's `Self` requirements,
because doing so would recursively trigger the same lookup again.
Fixes rdar://problem/124498054.
