Fix parenthesis for optional function as return type (#11004)

Co-authored-by: fisker Cheung <lionkay@gmail.com>

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;
+};
+```

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": {

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;
+};
+
+================================================================================
+`;

tests/format/flow/parenthesis-return-type/format.test.js

@@ -0,0 +1 @@
+runFormatTest(import.meta, ["flow"]);

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;
+};