Find a pair summing to a target, in a sorted array

You are given an array of integers sorted in non-decreasing order, and a target value. Return the indices of two distinct elements that sum to the target, or an empty result if no such pair exists.

Solve it in O(n) time and O(1) extra space.

Then answer the part that is actually being assessed: why can you safely discard a candidate when you move a pointer? An interviewer who hears "I move left forward if the sum is too small" and nothing else has learned that you memorised the shape.