cppcoreguidelines-rvalue-reference-param-not-moved false-positive for assignment operator?

[EugeneZelenko]

I may be wrong, but looks like cppcoreguidelines-rvalue-reference-param-not-moved produces false positive when assignment operator with rvalue is available (https://en.cppreference.com/w/cpp/string/basic_string/operator%3D):

#include <string>
#include <vector>

std::vector<std::string> tokens;

void function(std::string&& token)
{
    tokens.resize(tokens.size() + 1);
    tokens[tokens.size() - 1] = token;
}

GodBolt: https://godbolt.org/z/9zhvr75db

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: None (EugeneZelenko)

I may be wrong, but looks like `cppcoreguidelines-rvalue-reference-param-not-moved` produces false positive when assignment operator with rvalue is available (https://en.cppreference.com/w/cpp/string/basic_string/operator%3D):

#include <string>
#include <vector>

std::vector<std::string> tokens;

void function(std::string&& token)
{
    tokens.resize(tokens.size() + 1);
    tokens[tokens.size() - 1] = token;
}

GodBolt: https://godbolt.org/z/9zhvr75db

[keinflue]

The diagnostic is correct. A rvalue reference parameter should be moved from (not more than once) in a function, meaning that there should be a call to std::move on it (once). It should be

void function(std::string&& token)
{
    tokens.resize(tokens.size() + 1);
    tokens[tokens.size() - 1] = std::move(token);
}

Otherwise the assignment operator with const lvalue argument is used. The name of a variable used as an expression is always a lvalue in C++, even if the type of the variable is rvalue reference. std::move is necessary to make a rvalue expression from the lvalue expression token.