Halliday & Matthiessen (2014: 161, 163):
What about WH- / Predicator? There is always the possibility that the missing piece the speaker wishes to have supplied may be something that is expressed in the verb — an action, event, mental process or relation — and hence functioning as Predicator. But the WH- element cannot be conflated with the Predicator; there is no verb to what in English, so we cannot ask whatted he? Questions of this kind are realised as do + what (Complement), or what (Subject) + happen, and whatever had something done to it, or happen to it, comes in as an Adjunct, in the form of a prepositional phrase, usually with the preposition to.This is one kind of Adjunct that is almost never thematic, for obvious reasons – not only would it have to override a WH- element, but it is not functioning as a circumstantial element anyway.