Below code snippet triggers a compilation error error: cannot bind rvalue reference of type ‘std::__cxx11::basic_string&&’ to lvalue of type ‘std::__cxx11::basic_string’.

#include <iostream>

std::string&& foo(std::string&& str)
{
    return std::move(str);
}

decltype(auto) bar(std::string&& str)
{
    decltype(auto) r{foo(std::move(str))};
    return r;
}

int main()
{
    std::string str{"Hello, World!"};
    auto&& ret{bar(std::move(str))};
    return 0;
}

According to decltype(auto) rules the local variable r has std::string&& type. Where is an lvalue involved in the return sequence that is caught by the compiler?