fmt already supports containers. Does it not support nested ones? The docs aren't clear.

Even if not, this library would surely be best implemented in terms of fmt. You could define a utility class "dump" that just holds a reference to the object you want to format. Then you can use it like this:

    fmt:: print("{}", dump(foo));

Then define a formatter for dump that uses constexpr if to decide whether to format the underlying object directly or to use your custom iteration logic. That is more extensible (it can format anything that fmt can), it avoids macros, and it's more efficient (it can format directly into the target buffer).