TypeResolution: Stop resolving unqualified protocol type aliases to DependentMemberType in structural stage

This workaround is no longer needed because `TypeAliasType` is now
modeled using generic arguments (as opposed to a substitution map).
Previously, computing the substitution map in `StructuralTypeRequest` to
construct the resulting `TypeAliasType` could cause a request cycle
around generic signature computation.

lib/Sema/TypeCheckGeneric.cpp

@@ -1127,11 +1127,6 @@ Type StructuralTypeRequest::evaluate(Evaluator &evaluator,
                                               /*packElementOpener*/ nullptr)
           .resolveType(underlyingTypeRepr);
 
-  // Don't build a generic siganture for a protocol extension, because this
-  // request might be evaluated while building a protocol requirement signature.
-  if (parentDC->getSelfProtocolDecl())
-    return result;
-
   Type parent;
   if (parentDC->isTypeContext())
     parent = parentDC->getSelfInterfaceType();

lib/Sema/TypeCheckType.cpp

@@ -560,15 +560,17 @@ Type TypeResolution::resolveTypeInContext(TypeDecl *typeDecl,
     selfType = foundDC->getSelfInterfaceType();
 
     if (selfType->is<GenericTypeParamType>()) {
-      if (isa<ProtocolDecl>(typeDecl->getDeclContext())) {
-        if (isa<AssociatedTypeDecl>(typeDecl) ||
-            (isa<TypeAliasDecl>(typeDecl) &&
-             !cast<TypeAliasDecl>(typeDecl)->isGeneric() &&
-             !isSpecialized)) {
-          if (getStage() == TypeResolutionStage::Structural) {
-            return DependentMemberType::get(selfType, typeDecl->getName());
-          } else if (auto assocType = dyn_cast<AssociatedTypeDecl>(typeDecl)) {
-            typeDecl = assocType->getAssociatedTypeAnchor();
+      if (auto assocType = dyn_cast<AssociatedTypeDecl>(typeDecl)) {
+        if (getStage() == TypeResolutionStage::Structural) {
+          return DependentMemberType::get(selfType, typeDecl->getName());
+        }
+
+        typeDecl = assocType->getAssociatedTypeAnchor();
+      } else if (auto *aliasDecl = dyn_cast<TypeAliasDecl>(typeDecl)) {
+        if (isa<ProtocolDecl>(typeDecl->getDeclContext()) &&
+            getStage() == TypeResolutionStage::Structural) {
+          if (aliasDecl && !aliasDecl->isGeneric()) {
+            return adjustAliasType(aliasDecl->getStructuralType());
           }
         }
       }

test/type/explicit_existential_protocol_typealias.swift

@@ -0,0 +1,16 @@
+// RUN: %target-typecheck-verify-swift -target %target-swift-5.9-abi-triple -enable-upcoming-feature ExistentialAny
+
+// REQUIRES: swift_feature_ExistentialAny
+
+protocol P {
+  typealias PAlias1 = P
+
+  func f1() -> any PAlias1
+  func g1<T>(_: T) -> any PAlias1
+}
+extension P {
+  typealias PAlias2 = P
+
+  func f2() -> any PAlias2 {}
+  func g2<T>(_: T) -> any PAlias2 {}
+}