Fix Except not working with generic properties (#263)

source/except.d.ts

@@ -1,3 +1,34 @@
+import {IsEqual} from './internal';
+
+/**
+Filter out keys from an object.
+
+Returns `never` if `Exclude` is strictly equal to `Key`.
+Returns `never` if `Key` extends `Exclude`.
+Returns `Key` otherwise.
+
+@example
+```
+type Filtered = Filter<'foo', 'foo'>;
+//=> never
+```
+
+@example
+```
+type Filtered = Filter<'bar', string>;
+//=> never
+```
+
+@example
+```
+type Filtered = Filter<'bar', 'foo'>;
+//=> 'bar'
+```
+
+@see {Except}
+*/
+type Filter<KeyType, ExcludeType> = IsEqual<KeyType, ExcludeType> extends true ? never : (KeyType extends ExcludeType ? never : KeyType);
+
 /**
 Create a type from an object type without certain keys.
 
@@ -21,4 +52,6 @@ type FooWithoutA = Except<Foo, 'a' | 'c'>;
 
 @category Utilities
 */
-export type Except<ObjectType, KeysType extends keyof ObjectType> = Pick<ObjectType, Exclude<keyof ObjectType, KeysType>>;
+export type Except<ObjectType, KeysType> = {
+	[KeyType in keyof ObjectType as Filter<KeyType, KeysType>]: ObjectType[KeyType];
+};

source/includes.d.ts

@@ -1,13 +1,4 @@
-/**
-Returns a boolean for whether the two given types are equal.
-
-@link https://github.com/microsoft/TypeScript/issues/27024#issuecomment-421529650
-*/
-type IsEqual<T, U> =
-	(<G>() => G extends T ? 1 : 2) extends
-	(<G>() => G extends U ? 1 : 2)
-		? true
-		: false;
+import {IsEqual} from './internal';
 
 /**
 Returns a boolean for whether the given array includes the given item.

source/internal.d.ts

@@ -0,0 +1,11 @@
+/**
+Returns a boolean for whether the two given types are equal.
+
+@link https://github.com/microsoft/TypeScript/issues/27024#issuecomment-421529650
+@link https://stackoverflow.com/questions/68961864/how-does-the-equals-work-in-typescript/68963796#68963796
+*/
+export type IsEqual<T, U> =
+	(<G>() => G extends T ? 1 : 2) extends
+	(<G>() => G extends U ? 1 : 2)
+		? true
+		: false;

test-d/except.ts

@@ -3,3 +3,14 @@ import {Except} from '../index';
 
 declare const except: Except<{a: number; b: string}, 'b'>;
 expectType<{a: number}>(except);
+
+// Generic properties
+interface Example {
+	[key: string]: unknown;
+	foo: number;
+	bar: string;
+}
+
+const test: Except<Example, 'bar'> = {foo: 123, bar: 'asdf'};
+expectType<number>(test.foo);
+expectType<unknown>(test.bar);