From 599bad14158a7815430662f96ecdd6d2e1f0fe54 Mon Sep 17 00:00:00 2001
From: Christopher Chedeau <vjeuxx@gmail.com>
Date: Sat, 24 Jan 2026 13:03:35 -0800
Subject: [PATCH] Fix parenthesis for optional function as return type (#11004)

Co-authored-by: fisker Cheung <lionkay@gmail.com>
---
 changelog_unreleased/flow/11004.md            | 19 +++++++++++++
 .../parentheses/needs-parentheses.js          | 23 ++++++++++++++--
 .../__snapshots__/format.test.js.snap         | 27 +++++++++++++++++++
 .../parenthesis-return-type/format.test.js    |  1 +
 .../flow/parenthesis-return-type/optional.js  |  7 +++++
 5 files changed, 75 insertions(+), 2 deletions(-)
 create mode 100644 changelog_unreleased/flow/11004.md
 create mode 100644 tests/format/flow/parenthesis-return-type/__snapshots__/format.test.js.snap
 create mode 100644 tests/format/flow/parenthesis-return-type/format.test.js
 create mode 100644 tests/format/flow/parenthesis-return-type/optional.js

diff --git a/changelog_unreleased/flow/11004.md b/changelog_unreleased/flow/11004.md
new file mode 100644
index 000000000..eb9503928
--- /dev/null
+++ b/changelog_unreleased/flow/11004.md
@@ -0,0 +1,19 @@
+#### Fix parenthesis for optional function as return type (#11004 by @vjeux, @fisker)
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+const fun = (a: number): (?(string => string)) => {
+  return a > 0 ? s => `${s}: ${a}` : null;
+};
+
+// Prettier stable
+const fun = (a: number): ?(string) => string => {
+  return a > 0 ? (s) => `${s}: ${a}` : null;
+};
+
+// Prettier main
+const fun = (a: number): (?(string) => string) => {
+  return a > 0 ? (s) => `${s}: ${a}` : null;
+};
+```
diff --git a/src/language-js/parentheses/needs-parentheses.js b/src/language-js/parentheses/needs-parentheses.js
index 503524e83..8e5031e8e 100644
--- a/src/language-js/parentheses/needs-parentheses.js
+++ b/src/language-js/parentheses/needs-parentheses.js
@@ -558,12 +558,31 @@ function needsParentheses(path, options) {
       );
     case "InferTypeAnnotation":
     case "NullableTypeAnnotation":
-      return (
+      if (
         parent.type === "ArrayTypeAnnotation" ||
         (key === "objectType" &&
           (parent.type === "IndexedAccessType" ||
             parent.type === "OptionalIndexedAccessType"))
-      );
+      ) {
+        return true;
+      }
+
+      // If the return type is a nullable arrow function, then we need a paren
+      // otherwise the inner => can be assumed to be for the outer one.
+      if (
+        node.type === "NullableTypeAnnotation" &&
+        path.match(
+          undefined,
+          (node, key) =>
+            key === "typeAnnotation" && node.type === "TypeAnnotation",
+          (node, key) =>
+            key === "returnType" && node.type === "ArrowFunctionExpression",
+        )
+      ) {
+        return true;
+      }
+
+      break;
 
     case "ComponentTypeAnnotation":
     case "FunctionTypeAnnotation": {
diff --git a/tests/format/flow/parenthesis-return-type/__snapshots__/format.test.js.snap b/tests/format/flow/parenthesis-return-type/__snapshots__/format.test.js.snap
new file mode 100644
index 000000000..d00bc43b2
--- /dev/null
+++ b/tests/format/flow/parenthesis-return-type/__snapshots__/format.test.js.snap
@@ -0,0 +1,27 @@
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
+
+exports[`optional.js format 1`] = `
+====================================options=====================================
+parsers: ["flow"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+fun = (a: number): (?(string => string)) => {
+  return a > 0 ? s => \`\${s}: \${a}\` : null;
+};
+
+fun = (a: number): ?((string => string)) => {
+  return a > 0 ? s => \`\${s}: \${a}\` : null;
+};
+
+=====================================output=====================================
+fun = (a: number): (?(string) => string) => {
+  return a > 0 ? (s) => \`\${s}: \${a}\` : null;
+};
+
+fun = (a: number): (?(string) => string) => {
+  return a > 0 ? (s) => \`\${s}: \${a}\` : null;
+};
+
+================================================================================
+`;
diff --git a/tests/format/flow/parenthesis-return-type/format.test.js b/tests/format/flow/parenthesis-return-type/format.test.js
new file mode 100644
index 000000000..55c339883
--- /dev/null
+++ b/tests/format/flow/parenthesis-return-type/format.test.js
@@ -0,0 +1 @@
+runFormatTest(import.meta, ["flow"]);
diff --git a/tests/format/flow/parenthesis-return-type/optional.js b/tests/format/flow/parenthesis-return-type/optional.js
new file mode 100644
index 000000000..0bb086ad9
--- /dev/null
+++ b/tests/format/flow/parenthesis-return-type/optional.js
@@ -0,0 +1,7 @@
+fun = (a: number): (?(string => string)) => {
+  return a > 0 ? s => `${s}: ${a}` : null;
+};
+
+fun = (a: number): ?((string => string)) => {
+  return a > 0 ? s => `${s}: ${a}` : null;
+};