Re-thinking re-categorization: Is that really a complementizer?

Following Kayne’s (2014) argumentation that the complementizer that is indeed a
relative pronoun and with it the complement clause a special type of relative clause
(explicative, i.e. without a gap), the paper contributes to the discussion whether
that-complement clauses are also structurally relative clauses. One consequence of
this would be that that-clauses should not allow long wh-extraction, contrary to
what is observed in languages like English at first sight. However, the distribution
of resumptive pronouns in Alemannic, a Southern German dialect, indeed points
into that direction. Like the Celtic languages, Alemannic has a special particle for
relative clauses but can use the d-pronoun strategy as well. Both strategies can
be used to build long distance dependencies alike. But resumptive pronouns are
nearly obligatory with that-clauses in sharp contrast to those involving relative
clauses. This difference can find an explanation, if the particle-strategy creates a
genuine gap in the embedded clause whereas a that-complement clause is always
a full-fledged clause and the gap in it is only apparent, its appearance regulated by
outer-syntactic criteria.