Only look through explicit conformances.

Explanation: do not let global from implied conformances influence the isolation of the conforming type. The overall isolation should be only computed from the explicit conformances to enable global actor cut off with 'nonisolated' on protocols.

Author: simanerush

lib/Sema/TypeCheckConcurrency.cpp

@@ -5167,7 +5167,7 @@ getIsolationFromConformances(NominalTypeDecl *nominal) {
 
   std::optional<InferredActorIsolation> foundIsolation;
   for (auto conformance :
-       nominal->getLocalConformances(ConformanceLookupKind::NonStructural)) {
+       nominal->getLocalConformances(ConformanceLookupKind::OnlyExplicit)) {
 
     // Don't include inherited conformances. If a conformance is inherited
     // from a superclass, the isolation of the subclass should be inferred
@@ -5184,35 +5184,19 @@ getIsolationFromConformances(NominalTypeDecl *nominal) {
     }
 
     auto *proto = conformance->getProtocol();
-    auto inferredIsolation = getInferredActorIsolation(proto);
-    auto protoIsolation = inferredIsolation.isolation;
-    switch (protoIsolation) {
+    switch (auto protoIsolation = getActorIsolation(proto)) {
     case ActorIsolation::ActorInstance:
     case ActorIsolation::Unspecified:
+    case ActorIsolation::Nonisolated:
     case ActorIsolation::CallerIsolationInheriting:
     case ActorIsolation::NonisolatedUnsafe:
       break;
-    case ActorIsolation::Nonisolated:
-      if (inferredIsolation.source.kind == IsolationSource::Kind::Explicit) {
-        if (!foundIsolation) {
-          // We found an explicitly 'nonisolated' protocol.
-          foundIsolation = {
-              protoIsolation,
-              IsolationSource(proto, IsolationSource::Conformance)};
-        }
-        continue;
-      } else {
-        break;
-      }
 
     case ActorIsolation::Erased:
       llvm_unreachable("protocol cannot have erased isolation");
 
     case ActorIsolation::GlobalActor:
-      // If we encountered an explicit globally isolated conformance, allow it
-      // to override the nonisolated isolation kind.
-      if (!foundIsolation ||
-          conformance->getSourceKind() == ConformanceEntryKind::Explicit) {
+      if (!foundIsolation) {
         foundIsolation = {
           protoIsolation,
           IsolationSource(proto, IsolationSource::Conformance)