[Concurrency] Isolated static 'let's are not safe to access across actors.

(cherry picked from commit 8eab5fe)

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -512,6 +512,11 @@ static bool varIsSafeAcrossActors(const ModuleDecl *fromModule,
     if (var->getAttrs().hasAttribute<NonisolatedAttr>())
       return true;
 
+    // Static 'let's are initialized upon first access, so they cannot be
+    // synchronously accessed across actors.
+    if (var->isStatic())
+      return false;
+
     // If it's distributed, generally variable access is not okay...
     if (auto nominalParent = var->getDeclContext()->getSelfNominalTypeDecl()) {
       if (nominalParent->isDistributedActor())

test/Concurrency/Runtime/actor_assert_precondition_executor.swift

@@ -61,6 +61,15 @@ actor Someone {
   }
 }
 
+@MainActor
+struct TestStaticVar {
+  @MainActor static let shared = TestStaticVar()
+
+  init() {
+    checkPreconditionMainActor()
+  }
+}
+
 @main struct Main {
   static func main() async {
     let tests = TestSuite("AssertPreconditionActorExecutor")
@@ -78,6 +87,10 @@ actor Someone {
         await MainFriend().callCheckMainActor()
       }
 
+      tests.test("MainActor.assertIsolated() from static let initializer") {
+        _ = await TestStaticVar.shared
+      }
+
       #if !os(WASI)
       tests.test("precondition on actor (main): wrongly assume the main executor, from actor on other executor") {
         expectCrashLater(withMessage: "Incorrect actor executor assumption; Expected 'MainActor' executor.")