It is enforced by the compiler, what it doesn't do is guarantee it at runtime. I agree with you on the enums. Go doesn't have a reliable enum construct. But the argument that I, the library author, must validate and check against any possible type of "color" you, the program author, can come up with is just crazy talk. I'll provide a Color type, I'll even pre-define some colors for you, but if you send me a Color that's rainbow, I'll panic.

It literally isn't enforced by the compiler, because I can compile code which is invalid.

It’s not invalid because a string is a string. You didn’t invent a new type.