template <typename T> qReturnArg(T&&) is an unconstrained perfect
forwarder, and will "win" for everything for which `T&` is not an
equal match. e.g. a volatile T.
Restrict the deleted overload to actual rvalues. We don't need to use
`const volatile T&&`, because rvalues never bind to `T&`, we just need
to get off using a universal reference, and adding const does that.
As a drive-by, change the deleted overload's return type to void, to
mimic std::as_const(), the prototype for rvalue-deleted function
overloads.
Pick-to: 6.5
Change-Id: If4fbc311677b993488859b2c4e226b94daed71fe
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
c6dbae9550