[Concurrency] Fix a crash caused by misuse of isolated modifier

Adjust isolation checking to handle misused `isolated` attribute
and let attribute checker property diagnose it.

Resolves: rdar://148076903
Resolves: https://github.com/swiftlang/swift/issues/80363

lib/Sema/TypeCheckConcurrency.cpp

@@ -5046,7 +5046,13 @@ getIsolationFromAttributes(const Decl *decl, bool shouldDiagnose = true,
                            bool onlyExplicit = false) {
   // Look up attributes on the declaration that can affect its actor isolation.
   // If any of them are present, use that attribute.
-  auto isolatedAttr = decl->getAttrs().getAttribute<IsolatedAttr>();
+
+  // 'isolated` attribute in the declaration context currently applies
+  // only to `deinit` declarations, invalid uses are going to
+  // be diagnosed as part of attribute checking.
+  auto isolatedAttr = isa<DestructorDecl>(decl)
+                          ? decl->getAttrs().getAttribute<IsolatedAttr>()
+                          : nullptr;
   auto nonisolatedAttr = decl->getAttrs().getAttribute<NonisolatedAttr>();
   auto globalActorAttr = decl->getGlobalActorAttr();
   auto concurrentAttr = decl->getAttrs().getAttribute<ConcurrentAttr>();
@@ -5099,10 +5105,8 @@ getIsolationFromAttributes(const Decl *decl, bool shouldDiagnose = true,
   }
 
   // If the declaration is explicitly marked 'isolated', infer actor isolation
-  // from the context. Currently applies only to DestructorDecl
+  // from the context. Currently applies only to DestructorDecl.
   if (isolatedAttr) {
-    assert(isa<DestructorDecl>(decl));
-
     auto dc = decl->getDeclContext();
     auto selfTypeDecl = dc->getSelfNominalTypeDecl();
     std::optional<ActorIsolation> result;

test/Concurrency/issue-80363.swift

@@ -0,0 +1,47 @@
+// RUN: %target-typecheck-verify-swift
+
+// https://github.com/swiftlang/swift/issues/80363
+
+class C {}
+
+func testLocal() {
+  isolated let c = C()
+  // expected-error@-1 {{'isolated' may only be used on 'deinit' declarations}}
+  _ = c
+
+  isolated func test() {
+    // expected-error@-1 {{'isolated' may only be used on 'deinit' declarations}}
+  }
+}
+
+isolated var x: Int = 42
+// expected-error@-1 {{'isolated' may only be used on 'deinit' declarations}}
+
+isolated class Test {
+  // expected-error@-1 {{'isolated' may only be used on 'deinit' declarations}}
+}
+
+struct TestMembers {
+  isolated var q: String {
+    // expected-error@-1 {{'isolated' may only be used on 'deinit' declarations}}
+    get {
+      "ultimate question"
+    }
+
+    isolated set {
+      // expected-error@-1 {{expected 'get', 'set', 'willSet', or 'didSet' keyword to start an accessor definition}}
+    }
+  }
+
+  isolated let a: Int = 42
+  // expected-error@-1 {{'isolated' may only be used on 'deinit' declarations}}
+
+  isolated subscript(x: Int) -> Bool {
+    // expected-error@-1 {{'isolated' may only be used on 'deinit' declarations}}
+    get { true }
+  }
+
+  isolated func test() {
+    // expected-error@-1 {{'isolated' may only be used on 'deinit' declarations}}
+  }
+}

validation-test/compiler_crashers_2_fixed/ab146c5c26ca113.swift

@@ -1,3 +1,3 @@
 // {"signature":"getIsolationFromAttributes(swift::Decl const*, bool, bool)"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 isolated let a