c++c++14language-lawyerrvalue-referencexvalue

Value category of the subscript-expression on xvalue

From 5.2.1.1:

The expression E1[E2] is identical (by definition) to *((E1)+(E2)) [...] except that in the case of an array operand, the result is an lvalue if that operand is an lvalue and an xvalue otherwise.

However, with the code below:

struct S
{
    int arr[5];
};

int main()
{
    int &&r = S().arr[0];
}

both GCC and Clang complain about "rvalue reference cannot bind to lvalue int".

What have I misunderstood? As I understand S() is rvalue, S().arr is xvalue, so S().arr[0] should be xvalue as well and should be able to bind to rvalue refs.

Solution

  • You are correct, for the reason you cite. S().arr[0] is an xvalue as of DR 1213, so you should be able to bind an rvalue reference to it.

    This is gcc bug 79832. Note that clang HEAD compiles this fine.