Don't infer isolated conformances when the feature isn't enabled

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -7913,10 +7913,13 @@ ConformanceIsolationRequest::evaluate(Evaluator &evaluator, ProtocolConformance
     return ActorIsolation::forGlobalActor(globalActorType);
   }
 
-  // In a context where we are inferring @MainActor, if the conforming type
-  // is on the main actor, then the conformance is, too.
   auto dc = rootNormal->getDeclContext();
   ASTContext &ctx = dc->getASTContext();
+  if (!ctx.LangOpts.hasFeature(Feature::IsolatedConformances))
+    return ActorIsolation::forNonisolated(false);
+
+  // In a context where we are inferring @MainActor, if the conforming type
+  // is on the main actor, then the conformance is, too.
   auto nominal = dc->getSelfNominalTypeDecl();
   if (ctx.LangOpts.hasFeature(Feature::UnspecifiedMeansMainActorIsolated) &&
       nominal) {

lib/Sema/TypeCheckProtocol.cpp

@@ -3574,12 +3574,12 @@ ConformanceChecker::checkActorIsolation(ValueDecl *requirement,
     // Another way to address the issue is to mark the conformance as
     // isolated to the global actor or "@preconcurrency".
     if (Conformance->getSourceKind() == ConformanceEntryKind::Explicit &&
-        !Conformance->getIsolation().isGlobalActor() &&
+        !Conformance->isIsolated() &&
         !Conformance->isPreconcurrency() &&
         !suggestedPreconcurrencyOrIsolated &&
-        !requirementIsolation.isActorIsolated() &&
-        refResult.isolation.isGlobalActor()) {
-      if (Context.LangOpts.hasFeature(Feature::IsolatedConformances)) {
+        !requirementIsolation.isActorIsolated()) {
+      if (Context.LangOpts.hasFeature(Feature::IsolatedConformances) &&
+          refResult.isolation.isGlobalActor()) {
         std::string globalActorStr = "@" +
             refResult.isolation.getGlobalActor().getString();
         Context.Diags.diagnose(Conformance->getProtocolNameLoc(),