Free theorems from univalent reference types

We develop a denotational semantics for general reference types in an impredicative version of guarded homotopy type theory, an adaptation of synthetic guarded domain theory to Voevodsky's univalent foundations. We observe for the first time the profound impact of univalence on the denotational semantics of mutable state. Univalence automatically ensures that all computations are invariant under symmetries of the heap -- a bountiful source of free theorems. In particular, even the most simplistic univalent model enjoys many new program equivalences that do not hold when the same constructions are carried out in the universes of traditional set-level (extensional) type theory.