Fix broken format for ...infer T (#9044)

fisker · web-flow · commit 9a9dcbe12b95 · 2020-08-25T12:36:54.000+03:00
diff --git a/changelog_unreleased/typescript/pr-9044.md b/changelog_unreleased/typescript/pr-9044.md
@@ -0,0 +1,13 @@
+#### Fix broken format for `...infer T` ([#9044](https://github.com/prettier/prettier/pull/9044) by [@fisker](https://github.com/fisker))
+
+<!-- prettier-ignore -->
+```typescript
+// Input
+type Tail<T extends any[]> = T extends [infer U, ...infer R] ? R : never;
+
+// Prettier stable
+type Tail<T extends any[]> = T extends [infer U, ...(infer R)] ? R : never;
+
+// Prettier master
+type Tail<T extends any[]> = T extends [infer U, ...infer R] ? R : never;
+```
diff --git a/src/language-js/needs-parens.js b/src/language-js/needs-parens.js
@@ -417,8 +417,12 @@ function needsParens(path, options) {
         return true;
       }
     // fallthrough
-    case "TSTypeOperator":
     case "TSInferType":
+      if (node.type === "TSInferType" && parent.type === "TSRestType") {
+        return false;
+      }
+    // fallthrough
+    case "TSTypeOperator":
       return (
         parent.type === "TSArrayType" ||
         parent.type === "TSOptionalType" ||
diff --git a/tests/typescript/rest-type/__snapshots__/jsfmt.spec.js.snap b/tests/typescript/rest-type/__snapshots__/jsfmt.spec.js.snap
@@ -14,6 +14,31 @@ type TupleWithRest = [number, ...(1 extends 2 ? string[] : number[])];
 ================================================================================
 `;
 
+exports[`infer-type.ts format 1`] = `
+====================================options=====================================
+parsers: ["typescript"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+type Tail<T extends any[]> = T extends [infer U, ...infer R] ? R : never;
+
+type ReduceNextElement<
+  T extends readonly unknown[]
+> = T extends readonly [infer V, ...infer R] ? [V, R] : never
+
+=====================================output=====================================
+type Tail<T extends any[]> = T extends [infer U, ...infer R] ? R : never;
+
+type ReduceNextElement<T extends readonly unknown[]> = T extends readonly [
+  infer V,
+  ...infer R
+]
+  ? [V, R]
+  : never;
+
+================================================================================
+`;
+
 exports[`simple.ts format 1`] = `
 ====================================options=====================================
 parsers: ["typescript"]
diff --git a/tests/typescript/rest-type/infer-type.ts b/tests/typescript/rest-type/infer-type.ts
@@ -0,0 +1,5 @@
+type Tail<T extends any[]> = T extends [infer U, ...infer R] ? R : never;
+
+type ReduceNextElement<
+  T extends readonly unknown[]
+> = T extends readonly [infer V, ...infer R] ? [V, R] : never
