[Bug c++/71463] New: "ignoring attributes on template argument" in -O1 and above

["mail at milianw dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71463

            Bug ID: 71463
           Summary: "ignoring attributes on template argument" in -O1 and
                    above
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mail at milianw dot de
  Target Milestone: ---

I've seen this error with gcc (GCC) 6.1.1 20160501 and code like this:

~~~~~~~~~~~~~~
#include <cstdlib>

template<typename T> struct foo {};
foo<decltype(&::malloc)> emit_unexpected_warning;

int main() { return 0; }
~~~~~~~~~~~~~~

compiling it without optimizations shows no warning, but from -O1 and onwards I
get the unexpected warning on ignored template argument attributes:

~~~~~~~~~~~~~~~
$ g++ -std=c++11 -Wall -Wpedantic -O0 test.cpp
# no warnings
$ g++ -std=c++11 -Wall -Wpedantic -O1 test.cpp
test.cpp:9:24: warning: ignoring attributes on template argument âvoid*
(*)(size_t) throw () {aka void* (*)(long unsigned int) throw ()}â
[-Wignored-attributes]
 foo<decltype(&::malloc)> emit_unexpected_warning;
~~~~~~~~~~~~~~~

Clang does not emit any such warning. I believe it's a false-positive,
especially considering that it only shows up in -O1 and higher, but not -O0.