[Bug c++/55535] New: Call to default constructor with overloaded subscript operator is interpreted as declaring/using array

[Bug c++/55535] New: Call to default constructor with overloaded subscript operator is interpreted as declaring/using array

[niels at penneman dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55535

             Bug #: 55535
           Summary: Call to default constructor with overloaded subscript
                    operator is interpreted as declaring/using array
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: niels@penneman.org


gcc version:

Using built-in specs.
COLLECT_GCC=/usr/x86_64-pc-linux-gnu/gcc-bin/4.8.0-alpha20121125/g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/4.8.0-alpha20121125/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with:
/var/tmp/portage/sys-devel/gcc-4.8.0_alpha20121125/work/gcc-4.8-20121125/configure
--prefix=/usr --bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/4.8.0-alpha20121125
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.0-alpha20121125/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.8.0-alpha20121125
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.8.0-alpha20121125/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.8.0-alpha20121125/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.0-alpha20121125/include/g++-v4
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --disable-altivec
--disable-fixed-point --with-ppl --with-cloog --disable-ppl-version-check
--with-cloog-include=/usr/include/cloog-ppl --enable-lto --enable-nls
--without-included-gettext --with-system-zlib --enable-obsolete
--disable-werror --enable-secureplt --enable-multilib
--with-multilib-list=m32,m64 --enable-libmudflap --disable-libssp
--enable-libgomp
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/4.8.0-alpha20121125/python
--enable-checking=release --disable-libgcj --disable-libquadmath
--enable-languages=c,c++ --enable-shared --enable-threads=posix
--enable-__cxa_atexit --enable-clocale=gnu --enable-targets=all
--with-bugurl=http://bugs.gentoo.org/ --with-pkgversion='Gentoo
4.8.0_alpha20121125'
Thread model: posix
gcc version 4.8.0-alpha20121125 20121125 (experimental) (Gentoo
4.8.0_alpha20121125) 

====================

Command and output:
$ g++ -fsyntax-only -Wall -Wextra voodoo.cxx 
voodoo.cxx: In function ‘void func()’:
voodoo.cxx:12:20: error: size of array has non-integral type ‘Empty’
   (Voodoo()[Empty()]) | Empty();
                    ^
voodoo.cxx:12:20: error: ‘type name’ declared as function returning an array
voodoo.cxx:12:20: error: size of array has non-integral type ‘Empty’
voodoo.cxx:12:20: error: ‘type name’ declared as function returning an array

====================

Full source code of 'voodoo.cxx':

struct Empty {};
struct Voodoo
{
  Empty operator [](Empty);
};
void func()
{
  (Voodoo()[Empty()]);
}

====================

It looks like gcc sees the type name (call to default constructor) followed by
the subscript operator as using or declaring an array. Without the parentheses
around "Voodoo()[Empty()]" makes the error go away. I don't see why the
parentheses should cause functionally different behaviour.

Because of that, and because the code passes syntax checking with clang 3.1,
icc 13 and armcc, I assume it's just something odd in g++. I am however not
100% sure that the above code conforms to the C++ standard, simply because I'm
not sure where to start looking.

While the above example looks useless the code has been extracted and
simplified from a >100k LOC boost/spirit-based parser where it did make sense.

Compiling this code also fails on 4.5.4, 4.6.3 and 4.7.2.

Note: (ancient) bug #13503 came up in 'possible duplicates' and looks slightly
related
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13503

[Bug c++/55535] Call to default constructor with overloaded subscript operator is interpreted as declaring/using array

[niels at penneman dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55535

--- Comment #1 from Niels Penneman <niels at penneman dot org> 2012-11-29 21:07:07 UTC ---
Wrong copy/paste for output. Below is the correct compiler output.

$ g++ -fsyntax-only -Wall -Wextra voodoo.cxx 
voodoo.cxx: In function ‘void func()’:
voodoo.cxx:10:20: error: size of array has non-integral type ‘Empty’
   (Voodoo()[Empty()]);
                    ^
voodoo.cxx:10:20: error: ‘type name’ declared as function returning an array
voodoo.cxx:10:20: error: size of array has non-integral type ‘Empty’
voodoo.cxx:10:20: error: ‘type name’ declared as function returning an array

[Bug c++/55535] Call to default constructor with overloaded subscript operator is interpreted as declaring/using array

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55535

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-11-29 21:18:47 UTC ---
I think this is ultimately PR53434 or a very close relative.

[Bug c++/55535] Call to default constructor with overloaded subscript operator is interpreted as declaring/using array

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55535

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Dup. Fixed for 4.9.0.

*** This bug has been marked as a duplicate of bug 29234 ***