From 9d0aa1ec9d1a7c0fdbe993d9646a599942eef47f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicol=C3=B2=20Ribaudo?= <nicolo.ribaudo@gmail.com>
Date: Tue, 10 Aug 2021 19:09:23 +0200
Subject: [PATCH] Disallow `<T>(a => b)` when parsing Flow (#13645)

---
 packages/babel-parser/src/plugins/flow/index.js      | 12 ++++++------
 .../type-generics/wrong-arrow-parens-no-jsx/input.js |  1 +
 .../wrong-arrow-parens-no-jsx/options.json           |  5 +++++
 .../flow/type-generics/wrong-arrow-parens/input.js   |  1 +
 .../type-generics/wrong-arrow-parens/options.json    |  3 +++
 5 files changed, 16 insertions(+), 6 deletions(-)
 create mode 100644 packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens-no-jsx/input.js
 create mode 100644 packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens-no-jsx/options.json
 create mode 100644 packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens/input.js
 create mode 100644 packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens/options.json

diff --git a/packages/babel-parser/src/plugins/flow/index.js b/packages/babel-parser/src/plugins/flow/index.js
index a57a75acaee2..8774da67da5e 100644
--- a/packages/babel-parser/src/plugins/flow/index.js
+++ b/packages/babel-parser/src/plugins/flow/index.js
@@ -2842,17 +2842,17 @@ export default (superClass: Class<Parser>): Class<Parser> =>
             },
           );
 
+          // <T>(() => {});
           // <T>(() => {}: any);
-          if (
-            arrowExpression.type !== "ArrowFunctionExpression" &&
-            arrowExpression.extra?.parenthesized
-          ) {
-            abort();
-          }
+          if (arrowExpression.extra?.parenthesized) abort();
 
           // The above can return a TypeCastExpression when the arrow
           // expression is not wrapped in parens. See also `this.parseParenItem`.
+          // (<T>() => {}: any);
           const expr = this.maybeUnwrapTypeCastExpression(arrowExpression);
+
+          if (expr.type !== "ArrowFunctionExpression") abort();
+
           expr.typeParameters = typeParameters;
           this.resetStartLocationFromNode(expr, typeParameters);
 
diff --git a/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens-no-jsx/input.js b/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens-no-jsx/input.js
new file mode 100644
index 000000000000..11885587667e
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens-no-jsx/input.js
@@ -0,0 +1 @@
+<T>(a => b);
diff --git a/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens-no-jsx/options.json b/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens-no-jsx/options.json
new file mode 100644
index 000000000000..c85b34519fb2
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens-no-jsx/options.json
@@ -0,0 +1,5 @@
+{
+  "sourceType": "module",
+  "plugins": ["flow"],
+  "throws": "Expected an arrow function after this type parameter declaration. (1:0)"
+}
diff --git a/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens/input.js b/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens/input.js
new file mode 100644
index 000000000000..11885587667e
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens/input.js
@@ -0,0 +1 @@
+<T>(a => b);
diff --git a/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens/options.json b/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens/options.json
new file mode 100644
index 000000000000..560947fe5a7f
--- /dev/null
+++ b/packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens/options.json
@@ -0,0 +1,3 @@
+{
+  "throws": "Unterminated JSX contents. (1:3)"
+}