Alongside the existing ExpectTypeOf methods, a method to check for successful extension of types would be helpful. Perhaps a strict/branded version as well.

Proposed syntax:

expectTypeOf<5>().toExtend<number>();
expectTypeOf<unknown>().toExtend<any>();
expectTypeOf('abc').toExtend([{}]); // Typescript error

expectTypeOf<5>().toStrictExtend<number>();
expectTypeOf<unknown>().toStrictExtend<any>(); // Typescript error
expectTypeOf('abc').toStrictExtend([{}]); // Typescript error

I would be happy to submit a PR for this. Especially since the Extends and StrictExtends types are already implemented, just not directly exposed in the main method.

My current workaround is:

expectTypeOf<
     StrictExtends<Foo, Bar>
>().toBeEqual<true>();

Admittedly the grammar is a bit different (most methods start with toBe) so open to a renaming to match. toBeExtending...?