IsLiteral: Return false for tagged types (#886)

sindresorhus · Jun 7, 2024 · 587380c · 587380c
1 parent cabce98
commit 587380c
Show file tree

Hide file tree

Showing 2 changed files with 12 additions and 4 deletions.
diff --git a/source/is-literal.d.ts b/source/is-literal.d.ts
@@ -22,10 +22,12 @@ LiteralCheck<1, string>
 */
 type LiteralCheck<T, LiteralType extends Primitive> = (
 	IsNever<T> extends false // Must be wider than `never`
-		? [T] extends [LiteralType] // Must be narrower than `LiteralType`
-			? [LiteralType] extends [T] // Cannot be wider than `LiteralType`
-				? false
-				: true
+		? [T] extends [LiteralType & infer U] // Remove any branding
+			? [U] extends [LiteralType] // Must be narrower than `LiteralType`
+				? [LiteralType] extends [U] // Cannot be wider than `LiteralType`
+					? false
+					: true
+				: false
 			: false
 		: false
 );

diff --git a/test-d/is-literal.ts b/test-d/is-literal.ts
@@ -5,6 +5,7 @@ import type {
 	IsNumericLiteral,
 	IsBooleanLiteral,
 	IsSymbolLiteral,
+	Tagged,
 } from '../index';
 
 const stringLiteral = '';
@@ -65,3 +66,8 @@ type A2 = IsNumericLiteral;
 type A3 = IsBooleanLiteral;
 // @ts-expect-error
 type A4 = IsSymbolLiteral;
+
+// Tagged types should be false
+expectType<IsStringLiteral<Tagged<string, 'Tag'>>>(false);
+expectType<IsNumericLiteral<Tagged<number, 'Tag'>>>(false);
+expectType<IsBooleanLiteral<Tagged<boolean, 'Tag'>>>(false);