C. F. Keil and Franz Delitzsch on 2 Kings 20:9-10

2Ki 20:9-10 הצּל הלך: “the shadow is gone ten degrees, if it should go back ten degrees?” The rendering, visne umbram solarii decem gradibus progredi an ... regredi, which Maurer still gives after the Vulgate, vis an ut ascendat ... an ut revertatur, cannot be grammatically reconciled with the perfect הלך, and is merely a conjecture founded upon the answer of Hezekiah.Hitzig and Knobel would therefore read הלך, though without furnishing any proofs that the inf. abs. is used for the future in the first clause of a double question, especially if the ה interrog. is wanting, and there is no special emphasis upon the verbal idea.

According to this answer, “it is easy for the shadow to decline (i.e., to go farther down) ten degrees; no (sc., that shall not be a sign to me), but if the shadow turn ten degrees backwards,” Isaiah seems to have given the king a choice as to the sign, namely, whether the shadow should go ten degrees forward or backward. But this does not necessarily follow from the words quoted. Hezekiah may have understood the prophet’s words וגו הצּל הלך hypothetically: “has the shadow gone (advanced) ten degrees, whether it should,” etc.; and may have replied, the advance of the shadow would not be a sure sign to him, but only its going back.