SetNonNullable: Fix usage in type predicate context (#515)

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

source/set-non-nullable.d.ts

@@ -1,6 +1,3 @@
-import type {Except} from './except';
-import type {Simplify} from './simplify';
-
 /**
 Create a type that makes the given keys non-nullable, where the remaining keys are kept as is.
 
@@ -35,10 +32,8 @@ type AllNonNullable = SetNonNullable<Foo>;
 
 @category Object
 */
-export type SetNonNullable<BaseType, Keys extends keyof BaseType = keyof BaseType> =
-	Simplify<
-	// Pick just the keys that are readonly from the base type.
-	Except<BaseType, Keys> &
-	// Pick the keys that should be non-nullable from the base type and make them non-nullable.
-	{[Key in Keys]: NonNullable<BaseType[Key]>}
-	>;
+export type SetNonNullable<BaseType, Keys extends keyof BaseType = keyof BaseType> = {
+	[Key in keyof BaseType]: Key extends Keys
+		? NonNullable<BaseType[Key]>
+		: BaseType[Key];
+};

test-d/set-non-nullable.ts

@@ -16,3 +16,15 @@ expectType<{a: number; b?: string}>(variation3);
 // Fail if type changes even if non-nullable is right.
 declare const variation4: SetNonNullable<{a: number; b: string | undefined}, 'b'>;
 expectNotAssignable<{a: string; b: string}>(variation4);
+
+// Update all keys if `Keys` generic is not passed.
+declare const variation5: SetNonNullable<{a: number; b: string | undefined; c: boolean | null}>;
+expectType<{a: number; b: string; c: boolean}>(variation5);
+
+// Does not throw type error in type predicate contexts.
+type Variation6Config = {a: boolean | null; b: boolean | null};
+const variant6Fn = <TProp extends keyof Variation6Config>(
+	config: Variation6Config,
+	prop: TProp,
+): config is SetNonNullable<Variation6Config, TProp> => Boolean(config[prop]);
+expectNotAssignable<never>(variant6Fn); // Just to prevent unused error.