`isDsymbol` will return non-NULL if an `Object` is *exactly* a `Dsymbol`. It is in the same group with `isType` and `isExpression`.
On the other hand, `getDsymbol` is used to extracts `Dsymbol` from template instance argument, which is represented as Object in dmd code. So it takes an `Object` and returns `Dsymbol`, and usually used in the template argument processing. 

OK. I'll take over this issue.