define.c

    Someone changed the parser to build a TypeName node on CREATE
    FUNCTION in any  case.  As  a  side  effect,  ALL!  functions
    created  got  the  proretset  attribute  to  true. Thus for a
    SELECT the parser wrapped an Iter node around  the  Expr  and
    since  singleton  functions  set  isDone  the Iter returns no
    tuple up.