CODEWITHSUNDEEP
c++
Hui
Hui

Reputation: 591

decltype(auto) function return type does not deduce && types

If a function returns decltype(auto) and it returns a local variable of type int&&, why does the return type is int&?

If I cast the variable to its own type, then the return type is what I expect (int&&)

#include <utility>

namespace{
    auto i = 5;
    auto j = 5;

    decltype(auto) foo1(){
        int&& ret = std::move(i);
        return ret;
    }

    decltype(auto) foo2(){
        int&& ret = std::move(j);
        return static_cast<decltype(ret)>(ret);
    }
}

int main(){
    static_assert(std::is_same_v<decltype(foo1()),int&>);
    static_assert(std::is_same_v<decltype(foo2()),int&&>);
}

Upvotes: 6

Views: 287

Answers (1)

Brian Bi
Brian Bi

Reputation: 119154

This seems to be a bug in GCC. Since decltype(ret) is int&&, foo1 should have return type int&&. However, this immediately renders foo1 ill-formed, since a function that returns int&& cannot have its return value initialized from ret, which is an lvalue (you would need std::move to make it work properly). Note that Clang gets this right (see link in comments to the question).

Upvotes: 3

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