Go is a good example, too. Unlike C++ though, the distinction between something with dynamic dispatch (an interface) is a bit more explicit than in C++ (a class

Go is a good example, too. Unlike C++ though, the distinction between something with dynamic dispatch (an interface) is a bit more explicit than in C++ (a class with virtual methods, which may not all be virtual). That said, in your example, yeah: if you just pass a stock http.Client around, you lose any ability to mock that short of spinning up a dummy webserver.