fix(51245): Class with parameter decorator in arrow function causes "…

…convert to default export" refactoring failure (microsoft#51256)

* fix(51245): don't rely on parent nodes in formatting rules

* check existing parent node

src/services/formatting/rules.ts

@@ -732,10 +732,10 @@ namespace ts.formatting {
     }
 
     function nodeIsInDecoratorContext(node: Node): boolean {
-        while (isExpressionNode(node)) {
+        while (node && isExpression(node)) {
             node = node.parent;
         }
-        return node.kind === SyntaxKind.Decorator;
+        return node && node.kind === SyntaxKind.Decorator;
     }
 
     function isStartOfVariableDeclarationList(context: FormattingContext): boolean {

tests/cases/fourslash/refactorConvertExport_namedToDefault2.ts

@@ -0,0 +1,23 @@
+/// <reference path='fourslash.ts' />
+
+// @Filename: /a.ts
+/////*a*/export const f = () => {
+////    return class C {
+////        constructor(@Foo() param: any) { }
+////    }
+////}/*b*/
+////function Foo(...args: any[]): any {}
+
+goTo.select("a", "b");
+edit.applyRefactor({
+    refactorName: "Convert export",
+    actionName: "Convert named export to default export",
+    actionDescription: "Convert named export to default export",
+    newContent:
+`export default () => {
+    return class C {
+        constructor(@Foo() param: any) { }
+    }
+}
+function Foo(...args: any[]): any {}`,
+});