[Bug c++/85784] New: False positive with -Wunused-but-set-parameter

[samueljlunt at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85784

            Bug ID: 85784
           Summary: False positive with -Wunused-but-set-parameter
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: samueljlunt at gmail dot com
  Target Milestone: ---

Given the following functions:

#include <utility>
#include <tuple>

template <typename... Args>
auto foo(Args&&...)
{}

template <typename... Args, std::size_t... Indexes>
auto bar(std::tuple<Args&&...> args, std::index_sequence<Indexes...>)
{
    return foo(std::forward<Args>(std::get<Indexes>(args))...);
}

template <typename... Args>
auto bar(std::tuple<Args&&...> args)
{
    return bar(std::move(args), std::index_sequence_for<Args...>{});
}

A function call "bar()" will generate a -Wunused-but-set-parameter warning,
while any call with at least one argument will not generate a warning.

I am guessing that this has something to do with the fact that when "...Args"
is empty, "foo(std::forward<Args>(std::get<Indexes>(args))...)" is expanded to
"foo()", and "std::tuple<> args_" is not used. However, I don't see why this
would trigger "-Wunused-but-set-parameter" (since no assignment takes places
here) instead of "-Wunused".

Either way, this seemed like a false positive to me, since in the template
function, args is used. It is only in a specific instantiation of the template
that it is not used.

Also, I noticed making args an rvalue reference, i.e.
bar(std::tuple<Args&&...>&& args, std::index_sequence<Indexes...>), silences
the warning.

Here is an example on godbolt.org illustrating the warning:
https://godbolt.org/g/VaA979