  Optical lattice clocks with uncertainty and instability in the
$10^{-17}$-range and below have so far been demonstrated exclusively using
fermions. Here, we demonstrate a bosonic optical lattice clock with $3\times
10^{-18}$ instability and $2.0\times 10^{-17}$ accuracy, both values improving
on previous work by a factor 30. This was enabled by probing the clock
transition with an ultra-long interrogation time of 4 s, using the long
coherence time provided by a cryogenic silicon resonator, by careful
stabilization of relevant operating parameters, and by operating at low atom
density. This work demonstrates that bosonic clocks, in combination with highly
coherent interrogation lasers, are suitable for high-accuracy applications with
particular requirements, such as high reliability, transportability, operation
in space, or suitability for particular fundamental physics topics. As an
example, we determine the $^{88}\textrm{Sr} - ^{87}$Sr isotope shift with 12
mHz uncertainty.
