https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66712

            Bug ID: 66712
           Summary: [concepts] variadic concepts cause havoc
           Product: gcc
           Version: c++-concepts
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric.niebler at gmail dot com
  Target Milestone: ---

template <class T, class...Args>
concept bool _Constructible_ =
  requires (Args&&...args)
  {
    T{ ((Args&&)(args))... };
  };

template <class T, class...Args>
constexpr bool _constructible_() { return false; }

_Constructible_{T, ...Args...}
constexpr bool _constructible_() { return true; }

struct S
{
  S(int, char const *);
};

int main()
{
  static_assert(_constructible_<S, int, char const *>(), "");
}


Result:

temp.cpp: In function ‘int main()’:
temp.cpp:22:55: internal compiler error: tree check: expected record_type or
union_type or qual_union_type, have template_type_parm in lookup_base, at
cp/search.c:224
   static_assert(_constructible_<S, int, char const *>(), "");
                                                       ^

temp.cpp:22:55: internal compiler error: Aborted
g++: internal compiler error: Aborted (program cc1plus)
Please submit a full bug report,
with preprocessed source if appropriate.
See <http://gcc.gnu.org/bugs.html> for instructions.


The trouble with variadic concepts is a blocker for STL2.
>From gcc-bugs-return-491082-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Tue Jun 30 18:55:24 2015
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From: "steffen.muething at iwr dot uni-heidelberg.de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/66711] New: GCC does not correctly restore diagnostic state after pragma GCC diagnostic pop with -Werror
Date: Tue, 30 Jun 2015 18:55:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idf711

            Bug ID: 66711
           Summary: GCC does not correctly restore diagnostic state after
                    pragma GCC diagnostic pop with -Werror
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: steffen.muething at iwr dot uni-heidelberg.de
  Target Milestone: ---

Created attachment 35880
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id5880&actioníit
MWE, compile with "gcc -Wextra -Werror wrong-diagnostic-pop.c"

GCC 5.1.0 fails to correctly restore the diagnostics state after a "#pragma GCC
diagnostic pop" if the program was compiled with -Werror: Instead of emitting
an error, the compiler insteads issues a warning after the pragma.

I've attached a simple example file. When compiled with "-Wextra -Werror", I'd
expect the output:

"""
error.cc: In function 'int main()':
error.cc:9:10: error: comparison between signed and unsigned integer
expressions [-Werror=sign-compare]
   x += x < y ? 1 : 0;
          ^
error.cc:18:10: error: comparison between signed and unsigned integer
expressions [-Werror=sign-compare]
   x += x < y ? 1 : 0;
          ^
cc1plus: all warnings being treated as errors
"""

But I get the following:

"""
error.cc: In function 'int main()':
error.cc:9:10: error: comparison between signed and unsigned integer
expressions [-Werror=sign-compare]
   x += x < y ? 1 : 0;
          ^
error.cc:18:10: warning: comparison between signed and unsigned integer
expressions [-Wsign-compare]
   x += x < y ? 1 : 0;
          ^
cc1plus: all warnings being treated as errors
"""

The second diagnostic was turned into a warning. All older versions of GCC that
I have available and that support diagnostic push/pop (4.6.4, 4.7.4, 4.8.4,
4.9.2) handle this correctly and issue an error in both cases. Example output
from GCC 4.9.2:

"""
wrong-diagnostic-pop.c: In function 'main':
wrong-diagnostic-pop.c:9:10: error: comparison between signed and unsigned
integer expressions [-Werror=sign-compare]
   x += x < y ? 1 : 0;
          ^
wrong-diagnostic-pop.c:18:10: error: comparison between signed and unsigned
integer expressions [-Werror=sign-compare]
   x += x < y ? 1 : 0;
          ^
cc1: all warnings being treated as errors
"""

The problem is also present in the C++ frontend.