Value category of the subscript-expression on xvalue

Question

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.

@Someprogrammerdude I'm pretty sure it does, cf. the for(auto &&r : c) pattern that also works for std::vector<bool>. Edit: it does indeed. — Quentin
– Quentin, Commented Mar 12, 2017 at 17:32
@Someprogrammerdude rvalue references absolutely prolong lifetimes. — Barry
– Barry, Commented Mar 12, 2017 at 17:32
(rvalue) references prolong lifetimes, but only when a temporary value is assigned to them. In this case arr[0] returns a reference and the value is an array element, of an array which is a member variable of a temporary. So in this case r absolutely does dangle, just not for the reasons originally mentioned. So OP: while this is a compiler bug, this code is still a terrible idea. — Nir Friedman
– Nir Friedman, Commented Mar 12, 2017 at 17:37
@NirFriedman No, r would prolong the lifetime of S(): "The temporary to which the reference is bound or the temporary that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference" — Barry
– Barry, Commented Mar 12, 2017 at 17:39
@Barry No, you are incorrect. I don't know what else to say. Lifetime extension is not transitive. S is never assigned to a reference, so why would its lifetime be extended? Working example (had to change to const ref, because coliru's clang/gcc versions have the bug): coliru.stacked-crooked.com/a/c8f585b116449088. — Nir Friedman
– Nir Friedman, Commented Mar 12, 2017 at 17:42

Barry · Accepted Answer · 2017-03-12 17:32:05Z

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.

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