From e5d29f6e402ef6d2b173d555c471d2417401ab6a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hu=C3=A1ng=20J=C3=B9nli=C3=A0ng?= <jlhwung@gmail.com>
Date: Fri, 21 Jan 2022 18:27:34 -0500
Subject: [PATCH] fix: incorrect conciseBody lookahead (#14194)

---
 .../babel-parser/src/parser/expression.js     |  6 ++-
 .../input.js                                  |  1 +
 .../output.json                               | 40 +++++++++++++++++++
 .../invalid-arrow-before-in/input.js          |  1 +
 .../invalid-arrow-before-in/options.json      |  3 ++
 5 files changed, 49 insertions(+), 2 deletions(-)
 create mode 100644 packages/babel-parser/test/fixtures/es2015/arrow-functions/concise-body-binary-in-regression-14193/input.js
 create mode 100644 packages/babel-parser/test/fixtures/es2015/arrow-functions/concise-body-binary-in-regression-14193/output.json
 create mode 100644 packages/babel-parser/test/fixtures/es2015/arrow-functions/invalid-arrow-before-in/input.js
 create mode 100644 packages/babel-parser/test/fixtures/es2015/arrow-functions/invalid-arrow-before-in/options.json

diff --git a/packages/babel-parser/src/parser/expression.js b/packages/babel-parser/src/parser/expression.js
index a5671162f971..cab67f8f80cf 100644
--- a/packages/babel-parser/src/parser/expression.js
+++ b/packages/babel-parser/src/parser/expression.js
@@ -2373,8 +2373,10 @@ export default class ExpressionParser extends LValParser {
   ): N.ArrowFunctionExpression {
     this.scope.enter(SCOPE_FUNCTION | SCOPE_ARROW);
     let flags = functionFlags(isAsync, false);
-    // ConciseBody and AsyncConciseBody inherit [In]
-    if (!this.match(tt.bracketL) && this.prodParam.hasIn) {
+    // ConciseBody[In] :
+    //   [lookahead ≠ {] ExpressionBody[?In, ~Await]
+    //   { FunctionBody[~Yield, ~Await] }
+    if (!this.match(tt.braceL) && this.prodParam.hasIn) {
       flags |= PARAM_IN;
     }
     this.prodParam.enter(flags);
diff --git a/packages/babel-parser/test/fixtures/es2015/arrow-functions/concise-body-binary-in-regression-14193/input.js b/packages/babel-parser/test/fixtures/es2015/arrow-functions/concise-body-binary-in-regression-14193/input.js
new file mode 100644
index 000000000000..bb4eca7991df
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/es2015/arrow-functions/concise-body-binary-in-regression-14193/input.js
@@ -0,0 +1 @@
+() => [] in x
diff --git a/packages/babel-parser/test/fixtures/es2015/arrow-functions/concise-body-binary-in-regression-14193/output.json b/packages/babel-parser/test/fixtures/es2015/arrow-functions/concise-body-binary-in-regression-14193/output.json
new file mode 100644
index 000000000000..ee5545b32b1f
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/es2015/arrow-functions/concise-body-binary-in-regression-14193/output.json
@@ -0,0 +1,40 @@
+{
+  "type": "File",
+  "start":0,"end":13,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":13}},
+  "program": {
+    "type": "Program",
+    "start":0,"end":13,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":13}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start":0,"end":13,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":13}},
+        "expression": {
+          "type": "ArrowFunctionExpression",
+          "start":0,"end":13,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":13}},
+          "id": null,
+          "generator": false,
+          "async": false,
+          "params": [],
+          "body": {
+            "type": "BinaryExpression",
+            "start":6,"end":13,"loc":{"start":{"line":1,"column":6},"end":{"line":1,"column":13}},
+            "left": {
+              "type": "ArrayExpression",
+              "start":6,"end":8,"loc":{"start":{"line":1,"column":6},"end":{"line":1,"column":8}},
+              "elements": []
+            },
+            "operator": "in",
+            "right": {
+              "type": "Identifier",
+              "start":12,"end":13,"loc":{"start":{"line":1,"column":12},"end":{"line":1,"column":13},"identifierName":"x"},
+              "name": "x"
+            }
+          }
+        }
+      }
+    ],
+    "directives": []
+  }
+}
diff --git a/packages/babel-parser/test/fixtures/es2015/arrow-functions/invalid-arrow-before-in/input.js b/packages/babel-parser/test/fixtures/es2015/arrow-functions/invalid-arrow-before-in/input.js
new file mode 100644
index 000000000000..5515e71c9ccf
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/es2015/arrow-functions/invalid-arrow-before-in/input.js
@@ -0,0 +1 @@
+() => {} in x
diff --git a/packages/babel-parser/test/fixtures/es2015/arrow-functions/invalid-arrow-before-in/options.json b/packages/babel-parser/test/fixtures/es2015/arrow-functions/invalid-arrow-before-in/options.json
new file mode 100644
index 000000000000..93db7641c75b
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/es2015/arrow-functions/invalid-arrow-before-in/options.json
@@ -0,0 +1,3 @@
+{
+  "throws": "Unexpected token (1:9)"
+}
\ No newline at end of file