Prevent ForOfStatement from printing the forbidden sequence "for ( as…

…ync of"

packages/babel-generator/src/generators/statements.ts

@@ -82,16 +82,26 @@ export function WhileStatement(this: Printer, node: t.WhileStatement) {
   this.printBlock(node);
 }
 
-const buildForXStatement = function (op) {
-  return function (node: any) {
+const buildForXStatement = function (op: "in" | "of") {
+  return function (this: Printer, node: t.ForInStatement | t.ForOfStatement) {
     this.word("for");
     this.space();
-    if (op === "of" && node.await) {
+    if (op === "of" && (node as t.ForOfStatement).await) {
       this.word("await");
       this.space();
     }
     this.token("(");
+
+    // ECMAScript specifically forbids a for-of loop from starting with the
+    // token sequence "for ( async of", because it would be ambiguous with
+    // "for (async of => {};;)", so we need to add extra parentheses.
+    const leftNeedsParens =
+      op === "of" && t.isIdentifier(node.left) && node.left.name === "async";
+
+    if (leftNeedsParens) this.token("(");
     this.print(node.left, node);
+    if (leftNeedsParens) this.token(")");
+
     this.space();
     this.word(op);
     this.space();

packages/babel-generator/test/fixtures/edgecase/for-async-of/input.js

@@ -0,0 +1,7 @@
+for ((async) of []);
+
+for ((async) of async) async;
+
+for (\u0061sync of []);
+
+for (async in []);

packages/babel-generator/test/fixtures/edgecase/for-async-of/output.js

@@ -0,0 +1,7 @@
+for ((async) of []);
+
+for ((async) of async) async;
+
+for ((async) of []);
+
+for (async in []);