ArrayTail: Fix behaviour with non-tuple arrays (#1175)

source/array-tail.d.ts

@@ -1,5 +1,5 @@
 import type {If} from './if.d.ts';
-import type {IsArrayReadonly} from './internal/index.d.ts';
+import type {IfNotAnyOrNever, IsArrayReadonly} from './internal/index.d.ts';
 import type {UnknownArray} from './unknown-array.d.ts';
 
 /**
@@ -24,15 +24,18 @@ add3(4);
 
 @category Array
 */
-export type ArrayTail<TArray extends UnknownArray> =
+export type ArrayTail<TArray extends UnknownArray> = IfNotAnyOrNever<TArray,
 	TArray extends UnknownArray // For distributing `TArray`
 		? _ArrayTail<TArray> extends infer Result
 			? If<IsArrayReadonly<TArray>, Readonly<Result>, Result>
 			: never // Should never happen
-		: never; // Should never happen
+		: never
+>;
 
 type _ArrayTail<TArray extends UnknownArray> = TArray extends readonly [unknown?, ...infer Tail]
 	? keyof TArray & `${number}` extends never
-		? []
+		? TArray extends readonly []
+			? []
+			: TArray // Happens when `TArray` is a non-tuple array (e.g., `string[]`) or has a leading rest element (e.g., `[...string[], number]`)
 		: Tail
 	: [];

source/merge-deep.d.ts

@@ -8,12 +8,21 @@ import type {
 	UnknownArrayOrTuple,
 } from './internal/index.d.ts';
 import type {NonEmptyTuple} from './non-empty-tuple.d.ts';
-import type {ArrayTail} from './array-tail.d.ts';
+import type {ArrayTail as _ArrayTail} from './array-tail.d.ts';
 import type {UnknownRecord} from './unknown-record.d.ts';
 import type {EnforceOptional} from './enforce-optional.d.ts';
 import type {SimplifyDeep} from './simplify-deep.d.ts';
 import type {UnknownArray} from './unknown-array.d.ts';
 
+type Writable<TArray extends UnknownArray> = {-readonly [Key in keyof TArray]: TArray[Key]}; // TODO: Remove this
+
+// Using the default `ArrayTail` type causes issues, refer https://github.com/sindresorhus/type-fest/pull/1175/files#r2134694728.
+type ArrayTail<TArray extends UnknownArray> = TArray extends unknown // For distributing `TArray`
+	? keyof TArray & `${number}` extends never
+		? []
+		: Writable<_ArrayTail<TArray>>
+	: never; // Should never happen
+
 type SimplifyDeepExcludeArray<T> = SimplifyDeep<T, UnknownArray>;
 
 /**
@@ -86,7 +95,7 @@ type OmitRestTypeHelper<
 	Tail extends UnknownArrayOrTuple,
 	Type extends UnknownArrayOrTuple,
 	Result extends UnknownArrayOrTuple = [],
-> = Tail extends readonly []
+> = Tail extends []
 	? Result
 	: OmitRestType<Tail, [...Result, FirstArrayElement<Type>]>;
 

test-d/array-tail.ts

@@ -11,22 +11,14 @@ expectType<readonly []>(getArrayTail([] as const));
 expectType<readonly []>(getArrayTail(['a'] as const));
 expectType<readonly ['b', 'c']>(getArrayTail(['a', 'b', 'c'] as const));
 
-// Optional elements tests
-expectType<readonly [undefined, 'c']>(getArrayTail(['a', undefined, 'c'] as const));
-
-// Mixed optional/required
-type MixedArray = [string, undefined?, number?];
-expectType<[undefined?, number?]>(getArrayTail(['hello'] as MixedArray));
-
-// Optional numbers
-expectType<readonly [undefined, 3]>(getArrayTail([1, undefined, 3] as const));
-
-// Complex mixed case
-type ComplexArray = [string, boolean, number?, string?];
-expectType<[boolean, number?, string?]>(getArrayTail(['test', false] as ComplexArray));
+// Optional elements
+expectType<readonly [undefined, 'c'?]>(getArrayTail(['a', undefined, 'c'] as readonly ['a', undefined, 'c'?]));
+expectType<[undefined?, number?]>(getArrayTail(['hello'] as [string, undefined?, number?]));
+expectType<readonly [undefined?, 3?]>(getArrayTail([1, undefined, 3] as readonly [1, undefined?, 3?]));
+expectType<[boolean, number?, string?]>(getArrayTail(['test', false] as [string, boolean, number?, string?]));
 
 // All optional elements
-expectType<['b'?]>([] as ArrayTail<['a'?, 'b'?]>);
+expectType<['b'?]>({} as ArrayTail<['a'?, 'b'?]>);
 expectType<readonly [number?]>({} as ArrayTail<readonly [string?, number?]>);
 
 // Rest element
@@ -36,14 +28,23 @@ expectType<readonly [number, boolean?, ...string[]]>({} as ArrayTail<readonly [s
 // expectType<readonly [...string[], string, number]>({} as ArrayTail<readonly [...string[], string, number]>); // Rest & Required
 expectType<readonly [number, ...string[], boolean, bigint]>({} as ArrayTail<readonly [string, number, ...string[], boolean, bigint]>); // Required, Rest & Required
 
+// Labelled tuples
+expectType<[y: string]>({} as ArrayTail<[x: number, y: string]>);
+expectType<[bar: string, ...rest: boolean[]]>({} as ArrayTail<[foo: number, bar: string, ...rest: boolean[]]>);
+expectType<[...rest: boolean[], foo: number, bar: string]>({} as ArrayTail<[...rest: boolean[], foo: number, bar: string]>);
+
 // Union of tuples
-expectType<[] | ['b']>([] as ArrayTail<[] | ['a', 'b']>);
-expectType<readonly ['y'?] | ['b', ...string[]] | readonly []>([] as ArrayTail<readonly ['x'?, 'y'?] | ['a', 'b', ...string[]] | readonly string[]>);
+expectType<[] | ['b']>({} as ArrayTail<[] | ['a', 'b']>);
+expectType<readonly ['y'?] | ['b', ...string[]] | readonly string[]>({} as ArrayTail<readonly ['x'?, 'y'?] | ['a', 'b', ...string[]] | readonly string[]>);
 expectType<[number] | readonly [boolean, string?]>({} as ArrayTail<[string, number] | readonly [number, boolean, string?]>);
-expectType<readonly [number] | readonly []>({} as ArrayTail<readonly [string, number] | readonly string[]>);
+expectType<readonly [number] | readonly string[]>({} as ArrayTail<readonly [string, number] | readonly string[]>);
 
 // Non tuple arrays
-expectType<[]>({} as ArrayTail<string[]>);
-expectType<readonly []>({} as ArrayTail<readonly string[]>);
-expectType<[]>({} as ArrayTail<never[]>);
-expectType<[]>({} as ArrayTail<any[]>);
+expectType<string[]>({} as ArrayTail<string[]>);
+expectType<readonly string[]>({} as ArrayTail<readonly string[]>);
+expectType<never[]>({} as ArrayTail<never[]>);
+expectType<any[]>({} as ArrayTail<any[]>);
+
+// Boundary cases
+expectType<never>({} as ArrayTail<never>);
+expectType<any>({} as ArrayTail<any>);