Strange `IHasSourceInfo` interface

[asl]

The definition of IHasSourceInfo interface is quite strange:

class IHasSourceInfo {
 public:
    virtual SourceInfo getSourceInfo() const = 0;
    virtual cstring toString() const = 0;
    virtual ~IHasSourceInfo() {}
};

Essentially it demands the presence of toString method in addition to having a SourceInfo Why it is so?