There were two problems here.

First, shouldReceive('bar') mocks the bar function (and without ->andReturns(...), it will do nothing and return null), so I should use shouldHaveReceived('bar') at the end of my test instead.

But also, spy(Bar::class) creates mocks for all the functions in Bar (including bar), unless I explicitly create a partial mock, leading to this test that does exactly what I want:

    public function test_foobar()
    {
        $spy = $this->spy(Bar::class)->makePartial();
        $foo = $this->app->get(Foo::class);
        $this->assertEquals(42, $foo->foo());
        $spy->shouldHaveReceived('bar');
    }