Seems to have been introduced in commit c219d9b0a. I think there indeed was a "tupbasics.h" in some early drafts of that refactoring, but it didn't survive into the committed version.
bfba563 Fix erroneous cross-reference in comment.
src/include/access/htup_details.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)