From 204f9aa4d1d30abc2de03edaa0fd63bbe1c51134 Mon Sep 17 00:00:00 2001
From: fisker <lionkay@gmail.com>
Date: Tue, 25 Aug 2020 12:34:32 +0800
Subject: [PATCH] Remove parens around `TSInferType`

---
 changelog_unreleased/typescript/pr-9044.md    | 13 ++++++++++
 src/language-js/needs-parens.js               |  6 ++++-
 .../__snapshots__/jsfmt.spec.js.snap          | 25 +++++++++++++++++++
 tests/typescript/rest-type/infer-type.ts      |  5 ++++
 4 files changed, 48 insertions(+), 1 deletion(-)
 create mode 100644 changelog_unreleased/typescript/pr-9044.md
 create mode 100644 tests/typescript/rest-type/infer-type.ts

diff --git a/changelog_unreleased/typescript/pr-9044.md b/changelog_unreleased/typescript/pr-9044.md
new file mode 100644
index 000000000000..11d4399737f4
--- /dev/null
+++ b/changelog_unreleased/typescript/pr-9044.md
@@ -0,0 +1,13 @@
+#### Remove parens around `TSInferType` ([#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
index f9568168d6ba..b740e2b1afb0 100644
--- 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
index c45419e9c6ef..9c3a39999737 100644
--- 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
new file mode 100644
index 000000000000..3829b96b382d
--- /dev/null
+++ 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