IsStringLiteral: add new util

index.d.ts

@@ -101,6 +101,7 @@ export type {Replace} from './source/replace';
 export type {Includes} from './source/includes';
 export type {Get} from './source/get';
 export type {LastArrayElement} from './source/last-array-element';
+export type {IsStringLiteral} from './source/is-string-literal';
 
 // Miscellaneous
 export type {GlobalThis} from './source/global-this';

source/is-string-literal.d.ts

@@ -0,0 +1,23 @@
+/**
+Returns a boolean for whether a given string is a literal.
+
+@link https://github.com/sindresorhus/type-fest/issues/541
+
+Use-cases:
+- If you want to make sure that a given type is an explicit literal string, and not a generic string
+
+@example
+```
+import type {IsStringLiteral} from 'type-fest';
+
+IsStringLiteral<'red'>; // true
+IsStringLiteral<string>; // false
+```
+
+@category Template literal
+*/
+export type IsStringLiteral<T> = T extends string
+	? string extends T
+		? false
+		: true
+	: false;

test-d/is-string-literal.ts

@@ -0,0 +1,10 @@
+import {expectType} from 'tsd';
+import type {IsStringLiteral} from '../index';
+
+declare const test1: IsStringLiteral<'hello'>;
+declare const test2: IsStringLiteral<string>;
+declare const test3: IsStringLiteral<number>;
+
+expectType<true>(test1);
+expectType<false>(test2);
+expectType<false>(test3);