class RSpec::Mocks::ArgumentMatchers::SingletonMatcher

@private
`args.count == 1 && klass === args.first`.
easy to do comparisons like: ‘[klass::INSTANCE] == args` rather than
special case logic we need for some of these matchers, by making it
multiple instance since there is no state. It also facilities the
singleton instance of the matcher. There is no need to construct
Provides a `<klass name>::INSTANCE` constant for accessing a global
Intended to be subclassed by stateless, immutable argument matchers.