[Bug c++/79510] New: #pragma GCC diagnostic pop/push/ignored doesn't work as expected in template code

["p.chevalier at psenterprise dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79510

            Bug ID: 79510
           Summary: #pragma GCC diagnostic pop/push/ignored doesn't work
                    as expected in template code
           Product: gcc
           Version: 6.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: p.chevalier at psenterprise dot com
  Target Milestone: ---

Created attachment 40739
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40739&action=edit
main.cxx

If a line of code in a template function would trigger a warning, surrounding
it with #pragma push/ignored/pop doesn't work.

The warning still appears at the call site of the template function, where it
gets instantiated.

Using clang 3.9.1 with the exact same scenario doesn't exhibit the bug.

To reproduce, compile the following main.cxx:
--------------------------------------------------------------------------------
#include <iostream>

template <typename... T>
void print_tuple(T... args) {
#ifdef __clang__
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunused-variable"
#else
#ifdef __gcc__
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-variable"
#endif
#endif
    // We use the braced initialization list to expand the parameter pack
    // apply is unused on purpose
    auto apply = {(std::cout << args << " ", 0)...};
#ifdef __clang__
#pragma clang diagnostic pop
#else
#ifdef __gcc__
#pragma GCC diagnostic pop
#endif
#endif

    std::cout << std::endl;
}

int main() {
    print_tuple("clang++", "--std=c++14", "-Wall", "-Werror", "main.cxx", "✓");
    print_tuple("g++", "--std=c++14", "-Wall", "-Werror", "main.cxx", "✗");
    print_tuple("g++", "complains", "where", "print_tuple", "is", "called");
}
--------------------------------------------------------------------------------

g++ --std=c++14 -Wall -Werror main.cxx

produces the following output:

main.cxx: In instantiation of ‘void print_tuple(T ...) [with T = {const char*,
const char*, const char*, const char*, const char*, const char*}]’:
main.cxx:29:80:   required from here
main.cxx:16:10: error: unused variable ‘apply’ [-Werror=unused-variable]
     auto apply = {(std::cout << args << " ", 0)...};
          ^~~~~
cc1plus: all warnings being treated as errors