The Quran's Torah and Gospel: Phantoms of History?

One frequent explanation I hear regarding the problem of the Quran's claims about the Torah and Gospel is that the Torah and Gospel the Quran refers to are a different Torah and Gospel, which appeared at some point in the past, and then disappeared, without a trace. But is this how the Muslim sources describe the Torah and Gospel, or is this another evolutionary stage in the Muslim polemic of biblical corruption?