[Merovius]

> No, it's closer to template expansion, "when" is "executed" (for lack of a better word) during compilation. Inactive "when" branches are ignored by the code generator (and type checker, thus they can contain code that's invalid for given instantiation). There's no type-switch in the resulting Go code.

As I've now understood better what you mean, let me suggest a possible solution to your default-case conundrum: What about, if I don't want to allow the default-case, I just leave out the default case? And the compiler erroring out, if you pass in a type to a when-statement that doesn't have a valid case defined?