Clarification on the type needed in a `**` preceded parameter

When making a call to a function setting a parameter with `**` it is unclear what is the requirement from the underlying type. Is it required for the type to be `dict` or any type that implements `Mapping` is good enough?

Eg, the code below is valid when `foo` is a `dict`, is this also valid if `foo` is a `Mapping` that is not a `dict`?
```python
a = dict(**foo)
```
