I found the following sentence in The New York Times.
When people are in possession of their mental faculties, they are the experts on whether their lives are worth living. (SOURCE)
Now see the way this part - whether their lives are worth living. - is being used. It seems like it's a fused relative construction, and whether is being used as a relativizer.
But I have never known the word - whether - has ever been used as anything other than conjunction. So what is it here? Can anyone please explain the italic part in my quoted sentence?
I have found another sentence -
They are the experts on whether the output from these dams can easily and inexpensively be replaced, not fishery biologists. (SOURCE)
But this one is a person's comment on a blog post. But the former one is from a reputed source.
[For my own reference: some asides from StoneyB regarding the gap in the clause. HERE]