From the album ‘The Wall’

“Is There Anybody Out There?” is credited to Roger Waters as the writer, but the famous classical-style acoustic guitar section was not played by Gilmour or Waters.
It was played by session guitarist Joe DiBlasi. Gilmour reportedly tried the part but wasn’t happy with his classical-guitar technique, so DiBlasi was brought in to record it.
So: composition credit, Waters. The distinctive acoustic guitar performance, Joe DiBlasi.