fix formatting of union type as arrow function return type

changelog_unreleased/typescript/pr-6896.md

@@ -0,0 +1,22 @@
+#### Fix formatting of union type as arrow function return type ([#6896](https://github.com/prettier/prettier/pull/6896) by [@thorn0](https://github.com/thorn0))
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+export const getVehicleDescriptor = async (
+  vehicleId: string,
+): Promise<Collections.Parts.PrintedCircuitBoardAssembly['attributes'] | undefined> => {}
+
+// Prettier stable
+export const getVehicleDescriptor = async (
+  vehicleId: string
+): Promise<| Collections.Parts.PrintedCircuitBoardAssembly["attributes"]
+| undefined> => {};
+
+// Prettier master
+export const getVehicleDescriptor = async (
+  vehicleId: string
+): Promise<
+  Collections.Parts.PrintedCircuitBoardAssembly["attributes"] | undefined
+> => {};
+```

src/language-js/printer-estree.js

@@ -4587,10 +4587,13 @@ function printTypeParameters(path, options, print, paramsKey) {
         (n[paramsKey][0].type === "TSTypeReference" &&
           shouldHugType(n[paramsKey][0].typeName)) ||
         n[paramsKey][0].type === "NullableTypeAnnotation" ||
+        // See https://github.com/prettier/prettier/pull/6467 for the context.
         (greatGreatGrandParent &&
           greatGreatGrandParent.type === "VariableDeclarator" &&
           grandparent &&
           grandparent.type === "TSTypeAnnotation" &&
+          n[paramsKey][0].type !== "TSUnionType" &&
+          n[paramsKey][0].type !== "UnionTypeAnnotation" &&
           n[paramsKey][0].type !== "TSConditionalType" &&
           n[paramsKey][0].type !== "TSMappedType")));
 

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

@@ -1,5 +1,38 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`arrow-return-type.ts 1`] = `
+====================================options=====================================
+parsers: ["typescript"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+export const getVehicleDescriptor = async (
+  vehicleId: string,
+): Promise<
+  Descriptor | undefined
+> => {}
+
+
+export const getVehicleDescriptor = async (
+  vehicleId: string,
+): Promise<
+  Collections.Parts.PrintedCircuitBoardAssembly['attributes'] | undefined
+> => {}
+
+=====================================output=====================================
+export const getVehicleDescriptor = async (
+  vehicleId: string
+): Promise<Descriptor | undefined> => {};
+
+export const getVehicleDescriptor = async (
+  vehicleId: string
+): Promise<
+  Collections.Parts.PrintedCircuitBoardAssembly["attributes"] | undefined
+> => {};
+
+================================================================================
+`;
+
 exports[`object-method.ts 1`] = `
 ====================================options=====================================
 parsers: ["typescript"]

tests/typescript_generic/arrow-return-type.ts

@@ -0,0 +1,12 @@
+export const getVehicleDescriptor = async (
+  vehicleId: string,
+): Promise<
+  Descriptor | undefined
+> => {}
+
+
+export const getVehicleDescriptor = async (
+  vehicleId: string,
+): Promise<
+  Collections.Parts.PrintedCircuitBoardAssembly['attributes'] | undefined
+> => {}