From 287b11831b51490399e1d750f7e3f774d4d6e2b0 Mon Sep 17 00:00:00 2001
From: Stuart Cook <Zalathar@users.noreply.github.com>
Date: Mon, 26 Apr 2021 19:55:00 +1000
Subject: [PATCH] Prevent ForOfStatement from printing the forbidden sequence
 "for ( async of"

---
 .../babel-generator/src/generators/statements.ts | 16 +++++++++++++---
 .../test/fixtures/edgecase/for-async-of/input.js |  7 +++++++
 .../fixtures/edgecase/for-async-of/output.js     |  7 +++++++
 3 files changed, 27 insertions(+), 3 deletions(-)
 create mode 100644 packages/babel-generator/test/fixtures/edgecase/for-async-of/input.js
 create mode 100644 packages/babel-generator/test/fixtures/edgecase/for-async-of/output.js

diff --git a/packages/babel-generator/src/generators/statements.ts b/packages/babel-generator/src/generators/statements.ts
index 24ca879d9686..3fa8e33c6701 100644
--- a/packages/babel-generator/src/generators/statements.ts
+++ b/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();
diff --git a/packages/babel-generator/test/fixtures/edgecase/for-async-of/input.js b/packages/babel-generator/test/fixtures/edgecase/for-async-of/input.js
new file mode 100644
index 000000000000..934747c7b4f9
--- /dev/null
+++ b/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 []);
diff --git a/packages/babel-generator/test/fixtures/edgecase/for-async-of/output.js b/packages/babel-generator/test/fixtures/edgecase/for-async-of/output.js
new file mode 100644
index 000000000000..c5734919e906
--- /dev/null
+++ b/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 []);
\ No newline at end of file