Add RequireAtLeastOne type

Extracted from sindresorhus/electron-util#11

index.d.ts

@@ -84,3 +84,26 @@ const ab: Merge<Foo, Bar> = {a: 1, b: 2};
 ```
 */
 export type Merge<FirstType, SecondType> = Omit<FirstType, Extract<keyof FirstType, keyof SecondType>> & SecondType;
+
+/*
+Create a type that requires at least one of the given keys.
+
+@example
+```
+import {RequireAtLeastOne} from 'type-fest';
+
+type SystemMessages = {
+	macos?: string;
+	linux?: string;
+	windows?: string;
+	default?: string;
+};
+
+const messages: RequireAtLeastOne<SystemMessages, 'macos' | 'linux' | 'windows'> = {macos: 'hey', default: 'hello'};
+```
+*/
+export type RequireAtLeastOne<T, Keys extends keyof T = keyof T> =
+	Omit<T, Keys>
+	& {
+		[Key in Keys]-?: Required<Pick<T, Key>>
+	}[Keys];

index.test-d.ts

@@ -1,5 +1,6 @@
 /* eslint-disable import/no-unassigned-import */
 import './test/omit';
 import './test/merge';
+import './test/require-at-least-one';
 
 // TODO: Add negative tests. Blocked by: https://github.com/SamVerschueren/tsd-check/issues/2

test/require-at-least-one.ts

@@ -0,0 +1,22 @@
+import {RequireAtLeastOne} from '..';
+
+type SystemMessages = {
+	macos?: string;
+	linux?: string;
+	windows?: string;
+	default?: string;
+};
+
+type ValidMessages = RequireAtLeastOne<SystemMessages, 'macos' | 'linux' | 'windows'>;
+const test = (_: ValidMessages[]): void => {};
+
+test([
+	{macos: 'hey'},
+	{macos: 'hey', default: 'hello'},
+	{linux: 'sup', windows: 'hi'},
+	{macos: 'hey', linux: 'sup', windows: 'hi', default: 'hello'}
+
+	// Negative examples:
+	// {},
+	// {default: 'hello'},
+]);