...with 09d5b31e "loplugin:constantfunction:
store," which apparently over-eagerly reduced functionality that happened to be
constant in one specific implicit instantiation of a template, but not in
others.

(Plus, adapting the original code to silence false loplugin:staticmethods
warnings.)

Change-Id: I156df3aef1b3b0c9d36b2a54c57de29948749bde