LGTM for a merge, because getting these in sync is better than perfection.

I think that the this-escape can be avoided, but that it doesn't present a present danger and we can do so in a follow-up effort.

1. introduce package-private static BaseSetting#validate(T, Predicate<T>):

       void static validateValue(String name, T input, Predicate<T> validator) throws IllegalArgumentException {
           if (!validator.test(input)) {
               throw new IllegalArgumentException(String.format("Failed to validate setting `%s` with value: `%s`", name, input);
           }
       }

2. migrate the instance method BaseSetting#validate(T) and the validate-in-initializer calls to use the static method:

       public void validate(T input) throws IllegalArgumentException {
           validateValue(this.name, input, this.validator);
       }

       // protected BaseSetting(String name, T defaultValue, boolean strict, Predicate<T> validator) {
       // ...
           if (strict) {
               validateValue(name, input, validator);
           }
       // }

       // protected Coercible(...) {
           if (strict) {
               T coercedDefault = coerce(defaultValue);
               BaseSetting.validateValue(name, coercedDefault, validator);
               this.defaultValue = coercedDefault;
           } else {
               this.defaultValue = defaultValue;
           }
       // }