[Concurrency] Downgrade errors about actor-isolated witnesses to warnings until

Swift 6 when the witness is a non-Sendable 'let'.

Author: hborla

lib/Sema/TypeCheckProtocol.cpp

@@ -3233,6 +3233,19 @@ ConformanceChecker::checkActorIsolation(ValueDecl *requirement,
         witness->getInnermostDeclContext()).defaultDiagnosticBehavior();
   }
 
+  // If the witness is a non-Sendable 'let', compiler versions <= 5.10
+  // didn't diagnose this code, so downgrade the error to an warning
+  // until Swift 6.
+  if (auto *var = dyn_cast<VarDecl>(witness)) {
+    ActorReferenceResult::Options options = llvm::None;
+    isLetAccessibleAnywhere(
+        witness->getDeclContext()->getParentModule(),
+        var, options);
+    if (options.contains(ActorReferenceResult::Flags::Preconcurrency)) {
+      behavior = DiagnosticBehavior::Warning;
+    }
+  }
+
   // Complain that this witness cannot conform to the requirement due to
   // actor isolation.
   witness->diagnose(diag::actor_isolated_witness,

test/Concurrency/actor_isolation.swift

@@ -1543,6 +1543,15 @@ class OverridesNonsiolatedInit: SuperWithNonisolatedInit {
 // expected-note@+1 {{class 'NonSendable' does not conform to the 'Sendable' protocol}}
 class NonSendable {}
 
+protocol NonisolatedProtocol {
+  var ns: NonSendable { get } // expected-note {{'ns' declared here}}
+}
+
+actor ActorWithNonSendableLet: NonisolatedProtocol {
+  // expected-warning@+1 {{actor-isolated property 'ns' cannot be used to satisfy nonisolated protocol requirement; this is an error in Swift 6}}
+  let ns = NonSendable()
+}
+
 actor ProtectNonSendable {
   // expected-note@+1 2 {{property declared here}}
   let ns = NonSendable()