Return `T&` from assignment operators

Reason

The convention for operator overloads (especially on concrete types) is for operator=(const T&) to perform the assignment and then return (non-const) *this. This ensures consistency with standard-library types and follows the principle of "do as the ints do."

Note

Historically there was some guidance to make the assignment operator return const T&. This was primarily to avoid code of the form (a = b) = c -- such code is not common enough to warrant violating consistency with standard types.

Example

class Foo
{
 public:
    ...
    Foo& operator=(const Foo& rhs)
    {
      // Copy members.
      ...
      return *this;
    }
};

Enforcement

This should be enforced by tooling by checking the return type (and return value) of any assignment operator.