In what way is the operation common? We obviously would never say that there is never a use for such thing as there are clear edge cases where it is necessary,

In what way is the operation common? We obviously would never say that there is never a use for such thing as there are clear edge cases where it is necessary, but as jerf points out, it is probably not what you actually need in most cases. Even ignoring that in the most common case the map isn't the right structure to begin with, what even is the general case for the situations that remain? You mentioned the marshalling of arbitrary data case, but in that case you also have reflection details to worry about, and which you can optimize for with a custom implementation, and thus wouldn't likely use the built-in anyway. A sibling thread discussed the cache benefits of colocating the values with the keys if a map is exceedingly small, but as soon as the map is of any reasonable size the added overhead of the values is almost certainly going to blow the cache even where the keys alone might still fit. All of which is to say that the best approach is highly context dependent. How do you even begin to choose which is the general case if you were to include such a function?