LGTM is reporting a false-positive "Expression has no effect" warning. The expression LGTM is highlighting is this:

    add_global(varName);

The definition of add_global follows right after the constructor:

static auto add_global(std::string var) -> bool {
    return variables.insert(std::move(var)).second;
}

The variables variable is a static class variable which is being mutated by this function, which is most definitely an observable side-effect (which is, indeed, observed on parse.yy). There is no other add_global function accessible on this point, and no other variables variable accessible.

Here is the link to the alert page.