code: public boolean isConvertible(Type t, Type s, Warner warn) {
          if (t.hasTag(ERROR)) {
              return true;
          }
          boolean tPrimitive = t.isPrimitive();
          boolean sPrimitive = s.isPrimitive();
          if (tPrimitive == sPrimitive) {
              return isSubtypeUnchecked(t, s, warn);
          }
          if (!allowBoxing) return false;
          return tPrimitive
              ? isSubtype(boxedClass(t).type, s)
              : isSubtype(unboxedType(t), s);
      }
docs: Is t a subtype of or convertible via boxing/unboxing
  conversion to s?