feat: support readonly tuples as input to .each (#733)

packages/vitest/src/runtime/suite.ts

@@ -1,5 +1,5 @@
 import { format } from 'util'
-import type { File, RunMode, Suite, SuiteAPI, SuiteCollector, SuiteFactory, SuiteHooks, Test, TestAPI, TestFunction } from '../types'
+import type { File, MutableArray, RunMode, Suite, SuiteAPI, SuiteCollector, SuiteFactory, SuiteHooks, Test, TestAPI, TestFunction } from '../types'
 import { isObject, noop, toArray } from '../utils'
 import { createChainable } from './chain'
 import { collectTask, context, normalizeTest, runWithSuite } from './context'
@@ -145,8 +145,8 @@ function createSuite() {
     },
   ) as SuiteAPI
 
-  suite.each = <T>(cases: T[]) => {
-    return (name: string, fn: (...args: T extends any[] ? T : [T]) => void) => {
+  suite.each = <T>(cases: T[] | readonly T[]) => {
+    return (name: string, fn: (...args: T extends any[] | readonly any[] ? MutableArray<T> : [T]) => void) => {
       cases.forEach((i) => {
         const items = toArray(i) as any
         suite(formatTitle(name, items), () => fn(...items))
@@ -163,8 +163,8 @@ function createTest(fn: ((this: Record<'concurrent'| 'skip'| 'only'| 'todo'| 'fa
     fn,
   ) as TestAPI
 
-  test.each = <T>(cases: T[]) => {
-    return (name: string, fn: (...args: T extends any[] ? T : [T]) => void) => {
+  test.each = <T>(cases: T[] | readonly T[]) => {
+    return (name: string, fn: (...args: T extends any[] | readonly any[] ? MutableArray<T> : [T]) => void) => {
       cases.forEach((i) => {
         const items = toArray(i) as any
         test(formatTitle(name, items), () => fn(...items))

packages/vitest/src/types/general.ts

@@ -18,6 +18,8 @@ export type DeepMerge<F, S> = MergeInsertions<{
         : never;
 }>
 
+export type MutableArray<T extends readonly any[]> = { -readonly [k in keyof T]: T[k] }
+
 export interface Constructable {
   new (...args: any[]): any
 }

packages/vitest/src/types/tasks.ts

@@ -1,5 +1,5 @@
 import type { ChainableFunction } from '../runtime/chain'
-import type { Awaitable, ErrorWithDiff } from './general'
+import type { Awaitable, ErrorWithDiff, MutableArray } from './general'
 import type { UserConsoleLog } from '.'
 
 export type RunMode = 'run' | 'skip' | 'only' | 'todo'
@@ -44,7 +44,7 @@ export type Task = Test | Suite | File
 
 export type DoneCallback = (error?: any) => void
 export type TestFunction = (done: DoneCallback) => Awaitable<void>
-export type EachFunction = <T>(cases: T[]) => (name: string, fn: (...args: T extends any[] ? T : [T]) => void) => void
+export type EachFunction = <T>(cases: T[] | readonly T[]) => (name: string, fn: (...args: T extends any[] | readonly any[] ? MutableArray<T> : [T]) => void) => void
 
 export type TestAPI = ChainableFunction<
 'concurrent' | 'only' | 'skip' | 'todo' | 'fails',

test/core/test/each.test.ts

@@ -26,6 +26,20 @@ describe.each([
   })
 })
 
+describe.each([
+  [1, 'a', '1a'],
+  [1, 'b', '1b'],
+  [2, 'c', '2c'],
+] as const)('describe concatenate(%i, %s)', (a, b, expected) => {
+  test(`returns ${expected}`, () => {
+    // This will fail typechecking if const is not used and/or types for a,b are merged into a union
+    const typedA: number = a
+    const typedB: string = b
+
+    expect(`${typedA}${typedB}`).toBe(expected)
+  })
+})
+
 describe.each([
   { a: 1, b: 1, expected: 2 },
   { a: 1, b: 2, expected: 3 },