[Concurrency] Downgrade actor_isolated_mutating_func to a warning

in a narrow case that is accepted by compiler versions <=5.9

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -2760,7 +2760,8 @@ namespace {
         return false;
 
       bool result = false;
-      auto diagnoseIsolatedInoutState = [this, call, isPartialApply, &result](
+      bool downgradeToWarning = false;
+      auto diagnoseIsolatedInoutState = [&](
           ConcreteDeclRef declRef, SourceLoc argLoc) {
         auto decl = declRef.getDecl();
         auto isolation = getActorIsolationForReference(decl, getDeclContext());
@@ -2777,7 +2778,8 @@ namespace {
               ValueDecl *fnDecl = declRef->getDecl();
               ctx.Diags.diagnose(apply->getLoc(),
                                  diag::actor_isolated_mutating_func,
-                                 fnDecl->getName(), decl);
+                                 fnDecl->getName(), decl)
+                  .warnUntilSwiftVersionIf(downgradeToWarning, 6);
               result = true;
               return;
             }
@@ -2800,6 +2802,11 @@ namespace {
       };
 
       auto findIsolatedState = [&](Expr *expr) -> Expr * {
+        // This code used to not walk into InOutExpr, which allowed
+        // some invalid code to slip by in compilers <=5.9.
+        if (isa<InOutExpr>(expr))
+          downgradeToWarning = true;
+
         if (LookupExpr *lookup = dyn_cast<LookupExpr>(expr)) {
           if (isa<DeclRefExpr>(lookup->getBase())) {
             diagnoseIsolatedInoutState(lookup->getMember().getDecl(),

test/Concurrency/actor_inout_isolation.swift

@@ -307,3 +307,18 @@ actor ProtectArray {
     }
   }
 }
+
+extension Optional {
+  mutating func mutate() async {}
+}
+
+@available(SwiftStdlib 5.1, *)
+actor ProtectDictionary {
+  var dict: [Int: Int] = [:]
+
+  func invalid() async {
+    await dict[0].mutate()
+    // expected-warning@-1 {{cannot call mutating async function 'mutate()' on actor-isolated property 'dict'; this is an error in Swift 6}}
+    // expected-targeted-complete-warning@-2 {{passing argument of non-sendable type 'inout Optional<Int>' outside of actor-isolated context may introduce data races}}
+  }
+}