We already have a similar question about double faced cards transforming, however, this covers a single card transforming. Meld exiles a pair of cards and returns them "transformed" into a single card.
Say a player controls Hanweir Battlements and also controls a Chandra, Fire of Kaladesh enchanted with Metamorphic Alteration to be a copy of Hanweir Garrison. What happens when the meld effect tries to create Hanweir, the Writhing Township but can't?