[Concurrency] SE-0449: Implied conformances to nonisolated protocols make witnesses nonisolated

Even if the requirement is stated on an isolated protocol if the
conformance is implied by a nonisolated one all of the requirements
and witnesses should be nonisolated.

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -5138,6 +5138,11 @@ getIsolationFromWitnessedRequirements(ValueDecl *value) {
       std::tuple<ProtocolConformance *, ActorIsolation, ValueDecl *>;
   SmallVector<IsolatedRequirement, 2> isolatedRequirements;
   for (auto conformance : conformances) {
+    auto *implied =
+        conformance->getSourceKind() == ConformanceEntryKind::Implied
+            ? conformance->getImplyingConformance()
+            : nullptr;
+
     auto protocol = conformance->getProtocol();
     for (auto found : protocol->lookupDirect(value->getName())) {
       if (!isa<ProtocolDecl>(found->getDeclContext()))
@@ -5147,6 +5152,19 @@ getIsolationFromWitnessedRequirements(ValueDecl *value) {
       if (!requirement || isa<TypeDecl>(requirement))
         continue;
 
+      // The conformance implied by an explicitly stated nonisolated protocol
+      // makes all of the requirements nonisolated.
+      if (implied &&
+          implied->getSourceKind() == ConformanceEntryKind::Explicit) {
+        auto protocol = implied->getProtocol();
+        if (protocol->getAttrs().hasAttribute<NonisolatedAttr>()) {
+          isolatedRequirements.push_back(IsolatedRequirement{
+              implied, ActorIsolation::forNonisolated(/*unsafe=*/false),
+              requirement});
+          continue;
+        }
+      }
+
       auto requirementIsolation = getActorIsolation(requirement);
       switch (requirementIsolation) {
       case ActorIsolation::ActorInstance:

test/Concurrency/nonisolated_rules.swift

@@ -183,6 +183,27 @@ struct TestIsolated : NonisolatedWithMembers {
   }
 }
 
+@MainActor
+protocol Root {
+  func testRoot()
+}
+
+nonisolated protocol Child : Root {
+  func testChild()
+}
+
+struct TestDifferentLevels : Child {
+  func testRoot() {}
+  func testChild() {}
+  func testNonWitness() {}
+}
+
+nonisolated func testRequirementsOnMultipleNestingLevels(t: TestDifferentLevels) {
+  t.testRoot() // okay
+  t.testChild() // okay
+  t.testNonWitness() // okay
+}
+
 // MARK: - Extensions
 
 nonisolated extension GloballyIsolated {