The Wigner–Araki–Yanase (WAY) theorem establishes an important
constraint that conservation laws impose on quantum mechanical measurements.
We formulate the WAY theorem in the broader context of resource theories,
where one is constrained to a subset of quantum mechanical operations described
by a symmetry group. Establishing connections with the theory of quantum
state discrimination we obtain optimal unitaries describing the measurement
of arbitrary observables, explain how prior information can permit perfect
measurements that circumvent the WAY constraint, and provide a framework
that establishes a natural ordering on measurement apparatuses through a
decomposition into asymmetry and charge subsystems.