diff --git a/source/array-slice.d.ts b/source/array-slice.d.ts
index 045d41f3..da684932 100644
--- a/source/array-slice.d.ts
+++ b/source/array-slice.d.ts
@@ -7,6 +7,7 @@ import type {Not, TupleMin} from './internal/index.d.ts';
 import type {IsEqual} from './is-equal.d.ts';
 import type {And} from './and.d.ts';
 import type {ArraySplice} from './array-splice.d.ts';
+import type {IsNever} from './is-never.d.ts';
 
 /**
 Returns an array slice of a given range, just like `Array#slice()`.
@@ -60,13 +61,33 @@ export type ArraySlice<
 	Start extends number = never,
 	End extends number = never,
 > = Array_ extends unknown // To distributive type
-	? And<IsEqual<Start, never>, IsEqual<End, never>> extends true
-		? Array_
-		: number extends Array_['length']
-			? VariableLengthArraySliceHelper<Array_, Start, End>
-			: ArraySliceHelper<Array_, IsEqual<Start, never> extends true ? 0 : Start, IsEqual<End, never> extends true ? Array_['length'] : End>
+	? IsNever<Start> extends true
+		? IsNever<End> extends true
+			? _ArraySlice<Array_, Start, End>
+			: End extends unknown // To distribute `End`
+				? _ArraySlice<Array_, Start, End>
+				: never // Never happens
+		: IsNever<End> extends true
+			? Start extends unknown // To distribute `Start`
+				? _ArraySlice<Array_, Start, End>
+				: never // Never happens
+			: Start extends unknown // To distribute `Start`
+				? End extends unknown // To distribute `End`
+					? _ArraySlice<Array_, Start, End>
+					: never // Never happens
+				: never // Never happens
 	: never; // Never happens
 
+type _ArraySlice<
+	Array_ extends readonly unknown[],
+	Start extends number = 0,
+	End extends number = Array_['length'],
+> = And<IsEqual<Start, never>, IsEqual<End, never>> extends true
+	? Array_
+	: number extends Array_['length']
+		? VariableLengthArraySliceHelper<Array_, Start, End>
+		: ArraySliceHelper<Array_, IsEqual<Start, never> extends true ? 0 : Start, IsEqual<End, never> extends true ? Array_['length'] : End>;
+
 type VariableLengthArraySliceHelper<
 	Array_ extends readonly unknown[],
 	Start extends number,
diff --git a/source/string-slice.d.ts b/source/string-slice.d.ts
index cfb2a8b4..e125a99e 100644
--- a/source/string-slice.d.ts
+++ b/source/string-slice.d.ts
@@ -28,8 +28,8 @@ StringSlice<'abcde', -2, -1>;
 */
 export type StringSlice<
 	S extends string,
-	Start extends number = 0,
-	End extends number = StringToArray<S>['length'],
+	Start extends number = never,
+	End extends number = never,
 > = string extends S
 	? string
 	: ArraySlice<StringToArray<S>, Start, End> extends infer R extends readonly string[]
diff --git a/test-d/array-slice.ts b/test-d/array-slice.ts
index 5bc4b23a..146bf62c 100644
--- a/test-d/array-slice.ts
+++ b/test-d/array-slice.ts
@@ -38,3 +38,40 @@ expectType<ArraySlice<[1, 2, 3, ...string[], 4, 5], 0>>([1, 2, 3, ...(null! as s
 expectType<ArraySlice<[1, 2, 3, ...string[], 4, 5], 1>>([2, 3, ...(null! as string[]), 4, 5]);
 expectType<ArraySlice<[1, 2, 3, ...string[], 4, 5], 3>>([...(null! as string[]), 4, 5]);
 expectType<ArraySlice<[1, 2, 3, ...string[], 4, 5], 10>>([...(null! as string[]), 4, 5]);
+
+// Unions
+// Array is union
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], 0>>({} as [0, 1, 2] | ['a', 'b', 'c', 'd']); // Positive start, no end
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], -2>>({} as [1, 2] | ['c', 'd']); // Negative start, no end
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], 0, 2>>({} as [0, 1] | ['a', 'b']); // Positive start, positive end
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], -2, -1>>({} as [1] | ['c']); // Negative start, negative end
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], -3, 2>>({} as [0, 1] | ['b']); // Negative start, positive end
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], 1, -1>>({} as [1] | ['b', 'c']); // Positive start, negative end
+
+// Start is union
+expectType<ArraySlice<[0, 1, 2, 3], 1 | -2>>({} as [1, 2, 3] | [2, 3]); // Positive/Negative start, no end
+expectType<ArraySlice<[0, 1, 2, 3], 2 | -3, 3>>({} as [2] | [1, 2]); // Positive/Negative start, positive end
+expectType<ArraySlice<[0, 1, 2, 3], 0 | -2, -1>>({} as [2] | [0, 1, 2]); // Positive/Negative start, negative end
+
+// End is union
+expectType<ArraySlice<[0, 1, 2, 3], 0, 1 | -2>>({} as [0] | [0, 1]); // Positive start, positive/negative end
+expectType<ArraySlice<[0, 1, 2, 3], -2, 2 | -1>>({} as [] | [2]); // Negative start, positive/negative end
+
+// Array and start are unions
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], 1 | -1>>({} as [1, 2] | [2] | ['b', 'c', 'd'] | ['d']); // Positive/Negative start, no end
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], 1 | -2, 2>>({} as [1] | ['b'] | []); // Positive/Negative start, positive end
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], 0 | -2, -1>>({} as [0, 1] | [1] | ['a', 'b', 'c'] | ['c']); // Positive/Negative start, negative end
+
+// Array and end are unions
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], 2, 3 | -1>>({} as [2] | [] | ['c']); // Positive start, positive/negative end
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], -3, 3 | -2>>({} as [0, 1, 2] | [0] | ['b', 'c'] | ['b']); // Negative start, positive/negative end
+
+// Start and end are unions
+expectType<ArraySlice<[0, 1, 2, 3], -5 | 0 | 1, -2 | 0 | 3>>( // Positive/Negative start, positive/negative end
+	{} as [0, 1] | [0, 1, 2] | [] | [1] | [1, 2],
+);
+
+// Array, start and end are unions
+expectType<ArraySlice<[0, 1, 2] | ['a', 'b', 'c', 'd'], 1 | -4, 4 | -1>>( // Positive/Negative start, positive/negative end
+	{} as [1] | [1, 2] | [0, 1] | [0, 1, 2] | ['a', 'b', 'c', 'd'] | ['a', 'b', 'c'] | ['b', 'c'] | ['b', 'c', 'd'],
+);
diff --git a/test-d/string-slice.ts b/test-d/string-slice.ts
index fffb9428..20a609f0 100644
--- a/test-d/string-slice.ts
+++ b/test-d/string-slice.ts
@@ -15,3 +15,40 @@ expectType<StringSlice<'abcde', 100, 1>>('');
 expectType<StringSlice<string>>(null! as string);
 expectType<StringSlice<string, 1>>(null! as string);
 expectType<StringSlice<string, 1, 2>>(null! as string);
+
+// Unions
+// String is union
+expectType<StringSlice<'012' | 'abcd', 0>>({} as '012' | 'abcd'); // Positive start, no end
+expectType<StringSlice<'012' | 'abcd', -2>>({} as '12' | 'cd'); // Negative start, no end
+expectType<StringSlice<'012' | 'abcd', 0, 2>>({} as '01' | 'ab'); // Positive start, positive end
+expectType<StringSlice<'012' | 'abcd', -2, -1>>({} as '1' | 'c'); // Negative start, negative end
+expectType<StringSlice<'012' | 'abcd', -3, 2>>({} as '01' | 'b'); // Negative start, positive end
+expectType<StringSlice<'012' | 'abcd', 1, -1>>({} as '1' | 'bc'); // Positive start, negative end
+
+// Start is union
+expectType<StringSlice<'0123', 1 | -2>>({} as '123' | '23'); // Positive/Negative start, no end
+expectType<StringSlice<'0123', 2 | -3, 3>>({} as '2' | '12'); // Positive/Negative start, positive end
+expectType<StringSlice<'0123', 0 | -2, -1>>({} as '2' | '012'); // Positive/Negative start, negative end
+
+// End is union
+expectType<StringSlice<'0123', 0, 1 | -2>>({} as '0' | '01'); // Positive start, positive/negative end
+expectType<StringSlice<'0123', -2, 2 | -1>>({} as '' | '2'); // Negative start, positive/negative end
+
+// Array and start are unions
+expectType<StringSlice<'012' | 'abcd', 1 | -1>>({} as '12' | '2' | 'bcd' | 'd'); // Positive/Negative start, no end
+expectType<StringSlice<'012' | 'abcd', 1 | -2, 2>>({} as '1' | 'b' | ''); // Positive/Negative start, positive end
+expectType<StringSlice<'012' | 'abcd', 0 | -2, -1>>({} as '01' | '1' | 'abc' | 'c'); // Positive/Negative start, negative end
+
+// Array and end are unions
+expectType<StringSlice<'012' | 'abcd', 2, 3 | -1>>({} as '2' | '' | 'c'); // Positive start, positive/negative end
+expectType<StringSlice<'012' | 'abcd', -3, 3 | -2>>({} as '012' | '0' | 'bc' | 'b'); // Negative start, positive/negative end
+
+// Start and end are unions
+expectType<StringSlice<'0123', -5 | 0 | 1, -2 | 0 | 3>>( // Positive/Negative start, positive/negative end
+	{} as '01' | '012' | '' | '1' | '12',
+);
+
+// Array, start and end are unions
+expectType<StringSlice<'012' | 'abcd', 1 | -4, 4 | -1>>( // Positive/Negative start, positive/negative end
+	{} as '1' | '12' | '01' | '012' | 'abcd' | 'abc' | 'bc' | 'bcd',
+);