In fact, I muddled this thread up with a similar one (What - that)which I had contributed towards the same day or the previous one. Sorry. Anyway, here is the reference*.
Under the heading 15.5 “Wh-interrogative clauses”, the relevant passages are the following:
“Subordinate wh-interrogative clauses occur in the whole range of functions available to the nominal that-clause [ … ]
“The type of subordinate wh-interrogative clause that most closely resembles wh-questions is the indirect wh-question:
She asked me who would look after the baby.
Compare the direct question: She asked me, ‘Who will look after the baby?’
“But we can claim a chain of resemblance from the request for an answer to a question (as in the indirect question) through uncertainty about the answer (as in I’m not sure who will look after the baby), certainty about the answer (It’s obvious who will look after the baby), expressions of other mental states or processes about the answer (I found out who will look after the baby, It’s irrelevant who will look after the baby), and informing about the answer (I told you who would look after the baby). In all instances a question is explicitly or implicitly raised, a question focused on the wh-element.”
Perhaps it would have been less confusing to use Quirk’s name ("Subordinate wh-interrogative clauses") for this type of subordinate clause rather than “Indirect interrogative clauses”)? In fact I now see that Clark also suggests the term "indirect" to be confusing:
*QUIRK et al, “A Comprehensive Grammar of the English Language” (1992), p. 1051.I would even get rid of the term 'indirect'. It seems confusing.