[Bug c++/67376] New: Comparison with pointer to past-the-end of array fails inside constant expression

["ldionne.2 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67376

            Bug ID: 67376
           Summary: Comparison with pointer to past-the-end of array fails
                    inside constant expression
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldionne.2 at gmail dot com
  Target Milestone: ---

The following code fails to compile on GCC trunk:

    struct array {
        int elems_[1];
    };

    constexpr array a{{0}};
    static_assert(a.elems_ != a.elems_ + 1, "");

The error (formatted to fit the report) is

    [snip]: error: non-constant condition for static assertion
     static_assert(a.elems_ != a.elems_ + 1, "");
     ^
    [snip]: error: ‘(((const int*)(& a.array::elems_)) != 
                    (((const int*)(& a.array::elems_)) + 4u))’ 
            is not a constant expression
     static_assert(a.elems_ != a.elems_ + 1, "");
                            ^

It seems like GCC does not like the fact that we're comparing a past-the-end
pointer. But then again, that's weird because the following code works fine:

    constexpr int a[1] = {0};
    static_assert(a != a + 1, "");

Live example: http://melpon.org/wandbox/permlink/4QKMVN5vm3ePJcpY
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From: "juergen.reuter at desy dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/67377] New: gcc 6.0 fails to compile on Darwin 14
Date: Thu, 27 Aug 2015 23:05:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idg377

            Bug ID: 67377
           Summary: gcc 6.0 fails to compile on Darwin 14
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: juergen.reuter at desy dot de
  Target Milestone: ---

When compiling gcc (svn r227279) on Darwin 14.5.0 with Xcode 6.4, clang Apple
LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.5.0, I get the compile error:
test -f config.h || (rm -f stamp-h1 &&
/Applications/Xcode.app/Contents/Developer/usr/bin/make stamp-h1)
true "AR_FLAGS=rc" "CC_FOR_BUILD=gcc" "CFLAGS=-g " "CXXFLAGS=-g "
"CFLAGS_FOR_BUILD=-g -O2" "CFLAGS_FOR_TARGET=-g -O2"
"INSTALL=/opt/local/bin/ginstall -c" "INSTALL_DATA=/opt/local/bin/ginstall -c
-m 644" "INSTALL_PROGRAM=/opt/local/bin/ginstall -c"
"INSTALL_SCRIPT=/opt/local/bin/ginstall -c" "LDFLAGS=-Wl,-no_pie "
"LIBCFLAGS=-g -O2" "LIBCFLAGS_FOR_TARGET=-g -O2"
"MAKE=/Applications/Xcode.app/Contents/Developer/usr/bin/make"
"MAKEINFO=makeinfo --split-sizeP00000 --split-sizeP00000
--split-sizeP00000 " "PICFLAG=" "PICFLAG_FOR_TARGET=" "SHELL=/bin/sh"
"EXPECT=expect" "RUNTEST=runtest" "RUNTESTFLAGS=" "exec_prefix=/usr/local"
"infodir=/usr/local/share/info" "libdir=/usr/local/lib" "prefix=/usr/local"
"tooldir=/usr/local/x86_64-apple-darwin14.5.0" "AR=ar" "AS=as" "CC=gcc"
"CXX=g++ -std=gnu++98"
"LD=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld"
"LIBCFLAGS=-g -O2" "NM=nm" "PICFLAG=" "RANLIB=ranlib" "DESTDIR=" DO=all
multi-do # /Applications/Xcode.app/Contents/Developer/usr/bin/make
/Applications/Xcode.app/Contents/Developer/usr/bin/make  all-am
true  DO=all multi-do # /Applications/Xcode.app/Contents/Developer/usr/bin/make
test -f config.h || (rm -f stamp-h1 &&
/Applications/Xcode.app/Contents/Developer/usr/bin/make stamp-h1)
make[3]: Nothing to be done for `all'.
g++ -std=gnu++98 -fno-PIE -c  -DTARGET_NAME=\"x86_64-apple-darwin14.5.0\" -g
-DIN_GCC    -fno-strict-aliasing -fno-exceptions -fno-rtti
-fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings
-Wcast-qual -Wno-format -Wmissing-format-attribute -Woverloaded-virtual
-pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings
-fno-common  -DHAVE_CONFIG_H -I. -I. -I../../gcc -I../../gcc/.
-I../../gcc/../include -I./../intl -I../../gcc/../libcpp/include
-I/usr/local//include -I/usr/local//include -I/usr/local//include
-I../../gcc/../libdecnumber -I../../gcc/../libdecnumber/dpd -I../libdecnumber
-I../../gcc/../libbacktrace   -o toplev.o -MT toplev.o -MMD -MP -MF
./.deps/toplev.TPo ../../gcc/toplev.c
clang: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is
deprecated
[....]
../../gcc/toplev.c:1320:10: error: use of undeclared identifier
'flag_loop_block'
      || flag_loop_block
         ^
../../gcc/toplev.c:1321:10: error: use of undeclared identifier
'flag_loop_interchange'
      || flag_loop_interchange
         ^
../../gcc/toplev.c:1322:10: error: use of undeclared identifier
'flag_loop_strip_mine'
      || flag_loop_strip_mine
         ^

This has probably to do with changes from Yosemite 10.10.4->10.10.5, or with
Xcode 6.4 (quite unlikely) or any changes in gcc between r226713 and r227279.
When using r276713, toplevel.c compiles but then I'm stuck with:
The directory that should contain system headers does not exist:
  /usr/include
What puzzles me is that MAC OS X/clang tries to compile the C code in gcc by
means of g++ -std=gnu++98 and then issuing a warning that this behavior is
deprecated. I'll do a few check, but someone else should have been stumbled
over those things, too.