  Proton-proton interactions resulting in final states with two photons are
studied in a search for the signature of flavor-changing neutral current
interactions of top quarks (t) and Higgs bosons (H). The analysis is based on
data collected at a center-of-mass energy of 13 TeV with the CMS detector at
the LHC, corresponding to an integrated luminosity of 137 fb$^{-1}$. No
significant excess above the background prediction is observed. Upper limits on
the branching fractions ($\mathcal{B}$) of the top quark decaying to a Higgs
boson and an up (u) or charm quark (c) are derived through a binned fit to the
diphoton invariant mass spectrum. The observed (expected) 95% confidence level
upper limits are found to be 0.019 (0.031)% for $\mathcal B$(t $\to$ Hu) and
0.073 (0.051)% for $\mathcal{B}$(t $\to$ Hc). These are the strictest upper
limits yet determined.
