Exactly! If you walk the expanded form you have implementation-specific junk to deal with. Also, it may not be possible to map a form post-expansion to a source

Exactly! If you walk the expanded form you have implementation-specific junk to deal with. Also, it may not be possible to map a form post-expansion to a source location. If you walk the pre-expansion form you’re going to get bad results. There’s no winning here unless you have help from the implementation.