Only perform async refactor if it won't delete code

Author: Benjamin Lichtman

src/services/codefixes/convertToAsyncFunction.ts

@@ -2,11 +2,13 @@
 namespace ts.codefix {
     const fixId = "convertToAsyncFunction";
     const errorCodes = [Diagnostics.This_may_be_converted_to_an_async_function.code];
+    let codeActionSucceeded = true;
     registerCodeFix({
         errorCodes,
         getCodeActions(context: CodeFixContext) {
+            codeActionSucceeded = true;
             const changes = textChanges.ChangeTracker.with(context, (t) => convertToAsyncFunction(t, context.sourceFile, context.span.start, context.program.getTypeChecker(), context));
-            return [createCodeFixAction(fixId, changes, Diagnostics.Convert_to_async_function, fixId, Diagnostics.Convert_all_to_async_functions)];
+            return codeActionSucceeded ? [createCodeFixAction(fixId, changes, Diagnostics.Convert_to_async_function, fixId, Diagnostics.Convert_all_to_async_functions)] : [];
         },
         fixIds: [fixId],
         getAllCodeActions: context => codeFixAll(context, errorCodes, (changes, err) => convertToAsyncFunction(changes, err.file, err.start, context.program.getTypeChecker(), context)),
@@ -387,6 +389,10 @@ namespace ts.codefix {
         const hasArgName = argName && argName.identifier.text.length > 0;
         const shouldReturn = transformer.setOfExpressionsToReturn.get(getNodeId(parent).toString());
         switch (func.kind) {
+            case SyntaxKind.NullKeyword:
+            case SyntaxKind.UndefinedKeyword:
+                // do not produce a transformed statement for a null or undefined argument
+                break;
             case SyntaxKind.Identifier:
                 if (!hasArgName) break;
 
@@ -443,6 +449,9 @@ namespace ts.codefix {
                         return createNodeArray([createReturn(getSynthesizedDeepClone(funcBody) as Expression)]);
                     }
                 }
+            default:
+                // We've found a transformation body we don't know how to handle, so the refactoring should no-op to avoid deleting code.
+                codeActionSucceeded = false;
                 break;
         }
         return createNodeArray([]);