[Bug c++/68113] VLA+typeof+new -- confusing warning

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68113

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-11-28
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1
      Known to fail|                            |5.2.0, 6.0

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed with decltype instead of __typeof__.  The 'new decltype(a)' construct
is valid provided a is not a VLA.  The strange warning message is caused by gcc
not distinguishing 'new (int[n])' (a GCC extension) from 'new decltype(a)' in
the function that issues the warning (build_new_1 in cp/init.c).

$ cat x.cpp && /build/gcc-trunk-svn/gcc/xg++ -B /build/gcc-trunk-svn/gcc -S
-Wall -Wextra -Wpedantic -std=c++11 x.cpp 
int main ()
{
    int n = 1;
    int a [n];
    new decltype (a);
}
x.cpp: In function âint main()â:
x.cpp:4:13: warning: ISO C++ forbids variable length array âaâ [-Wvla]
     int a [n];
             ^

x.cpp:5:9: warning: non-constant array new length must be specified without
parentheses around the type-id [-Wvla]
     new decltype (a);
         ^~~~~~~~

Clang rejects the code because it doesn't allow VLAs in new expressions:

$ /build/llvm-trunk/bin/clang++ -Wall -Wextra -std=c++11 x.cpp
x.cpp:5:9: error: 'new' cannot allocate object of variably modified type
      'decltype(a)' (aka 'int [n]')
    new decltype (a);
        ^
1 error generated.