Incompletely quoting an API spec does nobody any good. Noted by
Paul Jungwirth. Looks like the discrepancy was my fault originally :-(
Discussion: https://postgr.es/m/CA+renyU_J8TU_d3Kr0PkuOgFbpypextendu7a+_d5NOfVdvDeA@mail.gmail.com
/* These are for invocation of a specifically named function with a
* directly-computed parameter list. Note that neither arguments nor result
- * are allowed to be NULL.
+ * are allowed to be NULL. Also, the function cannot be one that needs to
+ * look at FmgrInfo, since there won't be any.
*/
extern Datum DirectFunctionCall1Coll(PGFunction func, Oid collation,
Datum arg1);