Merge pull request #32424 from slavapestov/silly-assoc-type-default-cycle

Sema: Relax associated type default circularity check

Author: slavapestov

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1730,7 +1730,8 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       auto mentionsItself =
         defaultType.findIf([&](Type defaultType) {
           if (auto DMT = defaultType->getAs<DependentMemberType>()) {
-            return DMT->getAssocType() == AT;
+            return (DMT->getAssocType() == AT &&
+                    DMT->getBase()->isEqual(proto->getSelfInterfaceType()));
           }
           return false;
         });

test/decl/protocol/req/associated_type_default.swift

@@ -34,3 +34,9 @@ protocol P4 {
 }
 
 struct X4 : P4 {} // expected-error{{type 'X4' does not conform to protocol 'P4'}}
+
+// rdar://62355224 - circularity check for default type was over-eager
+protocol Seq {
+  associatedtype SubSeq: Seq
+  associatedtype Index = SubSeq.Index
+}