The following snippet fails on clang with :
"static_assert failed "`std::visit` requires the visitor to be exhaustive."
Both gcc and vc++ accept non-const lvalue references.

#include <iostream>
#include <variant>

struct A { int x = 33; };

struct Visitor
{
    void operator()(A &node) { std::cout << node.x; }
};
int main() 
{  
   std::variant<A> var = A{};
   std::visit(Visitor{}, var);
}