Exact: Fix handling of Opaque types (#525)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

Author: zorji

source/exact.d.ts

@@ -1,4 +1,5 @@
 import type {KeysOfUnion, ArrayElement, ObjectValue} from './internal';
+import type {Opaque} from './opaque';
 
 /**
 Create a type from `ParameterType` and `InputType` and change keys exclusive to `InputType` to `never`.
@@ -53,5 +54,7 @@ export type Exact<ParameterType, InputType> =
 	ParameterType extends unknown[] ? Array<Exact<ArrayElement<ParameterType>, ArrayElement<InputType>>>
 	// In TypeScript, Array is a subtype of ReadonlyArray, so always test Array before ReadonlyArray.
 		: ParameterType extends readonly unknown[] ? ReadonlyArray<Exact<ArrayElement<ParameterType>, ArrayElement<InputType>>>
-			: ParameterType extends object ? ExactObject<ParameterType, InputType>
-				: ParameterType;
+			// For Opaque types, internal details are hidden from public, so let's leave it as is.
+			: ParameterType extends Opaque<infer OpaqueType, infer OpaqueToken> ? ParameterType
+				: ParameterType extends object ? ExactObject<ParameterType, InputType>
+					: ParameterType;

test-d/exact.ts

@@ -1,4 +1,4 @@
-import type {Exact} from '../index';
+import type {Exact, Opaque} from '../index';
 
 { // Spec - string type
 	type Type = string;
@@ -334,3 +334,40 @@ import type {Exact} from '../index';
 		fn(input);
 	}
 }
+
+// Spec - special test case for Opaque type
+// @see https://github.com/sindresorhus/type-fest/issues/508
+{
+	type SpecialName = Opaque<string, 'special name'>;
+
+	type OnlyAcceptName = {
+		name: SpecialName;
+	};
+
+	const onlyAcceptNameImproved = <T extends Exact<OnlyAcceptName, T>>(arguments_: T) => arguments_;
+
+	onlyAcceptNameImproved({
+		// The error before the workaround:
+		// Error: Type 'SpecialName' is not assignable to type 'never'
+		name: 'name' as SpecialName,
+	});
+}
+
+// Spec - special test case for Opaque type
+// @see https://github.com/sindresorhus/type-fest/issues/508
+{
+	// Test for number Opaque type
+	type SpecialName = Opaque<number, 'special name'>;
+
+	type OnlyAcceptName = {
+		name: SpecialName;
+	};
+
+	const fn = <T extends Exact<OnlyAcceptName, T>>(arguments_: T) => arguments_;
+
+	fn({
+		// The error before the workaround:
+		// Error: Type 'SpecialName' is not assignable to type 'never'
+		name: 1 as SpecialName,
+	});
+}