[libcxx] [test] Allow a standard library that implements LWG 1203 in istream.rvalue/rvalue.pass.cpp

[BillyONeal]

Summary

If a standard library implements LWG 1203 (which I am implementing in MSVC++ right now) the return value of (istringstream() << A{}) is an rvalue, so it can't have an lvalue reference bound to it. Change this test to bind a forwarding reference instead which allows both standard behavior and LWG 1203 proposed behavior.

Note that LWG 1203 is presently not accepted.

[BillyONeal]

Committed r339214

(If there are changes requested I'll do them in a follow up commit)

[BillyONeal]

Adding ldionne as suggested by Eric.

[ldionne]

LGTM

test/std/input.output/iostream.format/input.streams/istream.rvalue/rvalue.pass.cpp

int main()
         testbuf<wchar_t> sb(L"   123");
         int i = 0;
         std::wistream(&sb) >> i;
         assert(i == 123);
     }
     { // test perfect forwarding
         assert(called == false);
         std::istringstream ss;
-        auto& out = (std::move(ss) >> A{});
+        auto&& out = (std::move(ss) >> A{});
         assert(&out == &ss);
         assert(called);
     }
 }