Hi I found that the header file where CompareType was actually defined did not match the header file described in gist.sgml. CompareType is currently defined at "src/include/access/cmptype.h" instead of "src/include/nodes/primnodes.h". This change was made in the commit below, but it seems that documentation update was missed.
https://github.com/postgres/postgres/commit/119fc30dd5bd918819b864107ddc8baac51f4d22 Regards
gist_doc.patch
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