[mirv]

By using util::sys() the compiler thinks you're trying to call a function [sys] in namespace util. The only thing it can match it against is a variable - hence the compiler output. If some variations of the compiler pick it up as a variable first, then it will still compile - the but the code is ambiguous in this respect and so should be changed anyway.

Also unless you mean to use things like operator overloading (+, -, *, /, etc) with a class, you should probably try and avoid the name "operator" for a function, as it may also cause a few complications.