TypeScript: Don’t breakup call expressions when the last argument is an arrow function with a simple return type (#6106)

CHANGELOG.unreleased.md

@@ -87,6 +87,29 @@ type G<T> = any;
 const myFunc = <T,>(arg1: G<T>) => false;
 ```
 
+### TypeScript: Don’t breakup call expressions when the last argument is an arrow function with a simple return type ([#6106] by [@brainkim])
+
+<!-- prettier-ignore -->
+```js
+Fixes [an edge-case](#6099) where we were splitting up call expressions containing arrow functions with simple return types.
+app.get("/", (req, res): void => {
+  res.send("Hello World!");
+});
+
+// Output (Prettier stable)
+app.get(
+  "/",
+  (req, res): void => {
+    res.send("Hello World!");
+  },
+);
+
+// Output (Prettier master)
+app.get("/", (req, res): void => {
+  res.send("Hello World!");
+});
+```
+
 ### JavaScript: Fix closure typecasts without spaces ([#6116] by [@jridgewell])
 
 Previously, a space was required between the `@type` and opening `{` of a closure typecast, or else the enclosing parenthesis would be removed. Closure itself does not require a space.
@@ -105,7 +128,9 @@ const v = /** @type{string} */ (value);
 
 [#5979]: https://github.com/prettier/prettier/pull/5979
 [#6115]: https://github.com/prettier/prettier/pull/6115
+[#6106]: https://github.com/prettier/prettier/pull/6106
 [#6116]: https://github.com/prettier/prettier/pull/6116
 [@jridgewell]: https://github.com/jridgewell
 [@jwbay]: https://github.com/jwbay
+[@brainkim]: https://github.com/brainkim
 [@sosukesuzuki]: https://github.com/sosukesuzuki

src/language-js/printer-estree.js

@@ -3716,7 +3716,20 @@ function couldGroupArg(arg) {
     arg.type === "TSAsExpression" ||
     arg.type === "FunctionExpression" ||
     (arg.type === "ArrowFunctionExpression" &&
-      !arg.returnType &&
+      // we want to avoid breaking inside composite return types but not simple keywords
+      // https://github.com/prettier/prettier/issues/4070
+      // export class Thing implements OtherThing {
+      //   do: (type: Type) => Provider<Prop> = memoize(
+      //     (type: ObjectType): Provider<Opts> => {}
+      //   );
+      // }
+      // https://github.com/prettier/prettier/issues/6099
+      // app.get("/", (req, res): void => {
+      //   res.send("Hello World!");
+      // });
+      (!arg.returnType ||
+        !arg.returnType.typeAnnotation ||
+        arg.returnType.typeAnnotation.type !== "TSTypeReference") &&
       (arg.body.type === "BlockStatement" ||
         arg.body.type === "ArrowFunctionExpression" ||
         arg.body.type === "ObjectExpression" ||

tests/typescript_arrow/__snapshots__/jsfmt.spec.js.snap

@@ -18,6 +18,10 @@ const foo = (x:string):void => (
   )
 );
 
+app.get("/", (req, res): void => {
+  res.send("Hello world");
+});
+
 =====================================output=====================================
 const bar = (...varargs: any[]) => {
   console.log(varargs);
@@ -25,5 +29,9 @@ const bar = (...varargs: any[]) => {
 
 const foo = (x: string): void => bar(x, () => {}, () => {});
 
+app.get("/", (req, res): void => {
+  res.send("Hello world");
+});
+
 ================================================================================
 `;

tests/typescript_arrow/arrow_regression.js

@@ -9,3 +9,7 @@ const foo = (x:string):void => (
     () => {}
   )
 );
+
+app.get("/", (req, res): void => {
+  res.send("Hello world");
+});