#48650 - Allow arbitrary TS syntax in comments in JS files.

src/compiler/diagnosticMessages.json

@@ -6885,6 +6885,10 @@
         "category": "Error",
         "code": 8039
     },
+    "JavaScript syntax can not be used inside of a TS comment": {
+        "category": "Error",
+        "code": 8040
+    },
 
     "Declaration emit for this file requires using private name '{0}'. An explicit type annotation may unblock declaration emit.": {
         "category": "Error",

src/compiler/parser.ts

@@ -1251,6 +1251,18 @@ export function forEachChild<T>(node: Node, cbNode: (node: Node) => T | undefine
     return fn === undefined ? undefined : fn(node, cbNode, cbNodes);
 }
 
+/**
+ * The definition of a leaf node isn't super precise here.
+ * For example, an empty object could be thought of as a leaf node since it doesn't have any children,
+ * but this function would claim that it isn't a leaf node.
+ * For the purposes of how this function gets used,
+ * what's important is that it won't ever claim that a node that has children is a leaf node.
+ * Other types of minor errors are fine.
+ */
+export function isLeafNode(node: Node) {
+    return (forEachChildTable as Record<SyntaxKind, ForEachChildFunction<any>>)[node.kind] === undefined;
+}
+
 /**
  * Invokes a callback for each child of the given node. The 'cbNode' callback is invoked for all child nodes
  * stored in properties. If a 'cbNodes' callback is specified, it is invoked for embedded arrays; additionally,
@@ -5241,6 +5253,7 @@ namespace Parser {
             }
         }
 
+        const firstTokenPrecededByOpeningTSComment = (scanner.getTokenFlags() & TokenFlags.enteringTSComment) !== 0;
         const first = token();
         const second = nextToken();
 
@@ -5327,7 +5340,7 @@ namespace Parser {
             }
 
             // JSX overrides
-            if (languageVariant === LanguageVariant.JSX) {
+            if (languageVariant === LanguageVariant.JSX && !firstTokenPrecededByOpeningTSComment) {
                 const isArrowFunctionInJsx = lookAhead(() => {
                     parseOptional(SyntaxKind.ConstKeyword);
                     const third = nextToken();
@@ -6531,7 +6544,7 @@ namespace Parser {
     }
 
     function parseTypeArgumentsInExpression() {
-        if ((contextFlags & NodeFlags.JavaScriptFile) !== 0) {
+        if ((contextFlags & NodeFlags.JavaScriptFile) !== 0 && (scanner.getTokenFlags() & TokenFlags.enteringTSComment) === 0) {
             // TypeArguments must not be parsed in JavaScript files to avoid ambiguity with binary operators.
             return undefined;
         }