[CodeCompletion] Avoid crash for not recommended item without a decl

Generating diagnostics for "not recommended" items requires an
associated declaration. However, cases like this:

  func test(fn: (Int) async -> Void) {
    fn(#^HERE^#)
  }

For the function call pattern item, there's no associated decl because
the callee is an expression. Ideally it should emit a diagnostic, but
for now, to avoid the crash, don't emit diagnostics unless the item has
the associated decl.

rdar://95306033

Author: rintaro

lib/IDE/CodeCompletionResultBuilder.cpp

@@ -87,8 +87,11 @@ CodeCompletionResult *CodeCompletionResultBuilder::takeResult() {
       CodeCompletionDiagnosticSeverity::None;
   NullTerminatedStringRef ContextFreeDiagnosticMessage;
   if (ContextFreeNotRecReason != ContextFreeNotRecommendedReason::None) {
+    assert(AssociatedDecl != nullptr &&
+           "There should be no case where ContextFreeNotRecReason != None && "
+           "AssociatedDecl == nulptr");
     // FIXME: We should generate the message lazily.
-    if (const auto *VD = dyn_cast<ValueDecl>(AssociatedDecl)) {
+    if (const auto *VD = dyn_cast_or_null<ValueDecl>(AssociatedDecl)) {
       CodeCompletionDiagnosticSeverity severity;
       SmallString<256> message;
       llvm::raw_svector_ostream messageOS(message);
@@ -169,7 +172,16 @@ CodeCompletionResult *CodeCompletionResultBuilder::takeResult() {
     NullTerminatedStringRef ContextualDiagnosticMessage;
     if (ContextualNotRecReason != ContextualNotRecommendedReason::None) {
       // FIXME: We should generate the message lazily.
-      if (const auto *VD = dyn_cast<ValueDecl>(AssociatedDecl)) {
+      //
+      // NOTE(rdar://95306033): dyn_cast_or_null because 'nullptr' happens in
+      // cases like:
+      //
+      //   func test(fn: () async -> Void) { fn(#HERE#) }
+      //
+      // In this case, it's 'InvalidAsyncContext' but there's no associated decl
+      // because the callee is a random expression.
+      // FIXME: Emit a diagnostic even without an associated decl.
+      if (const auto *VD = dyn_cast_or_null<ValueDecl>(AssociatedDecl)) {
         CodeCompletionDiagnosticSeverity severity;
         SmallString<256> message;
         llvm::raw_svector_ostream messageOS(message);

test/IDE/complete_diagnostics_concurrency.swift

@@ -30,3 +30,11 @@ func testActor(obj: MyActor) async {
 // ACTOR-DAG: Decl[InstanceMethod]/CurrNominal/NotRecommended: receiveNonSendable({#arg: MyNonSendable#})[' async'][#Void#]; name=receiveNonSendable(arg:); diagnostics=warning:actor-isolated 'receiveNonSendable(arg:)' should only be referenced from inside the actor{{$}}
 // ACTOR: End completions
 }
+
+func testClosure(obj: (Int) async -> Void) {
+  obj(#^CLOSURE_CALL^#)
+// FIXME: Emit diagnostics
+// CLOSURE_CALL: Begin completions
+// CLOSURE_CALL-DAG: Pattern/CurrModule/Flair[ArgLabels]/NotRecommended: ['(']{#Int#}[')'][' async'][#Void#]; name={{$}}
+// CLOSURE_CALL: End completions
+}