[Bug c++/87607] New: Cannot use fold expression in static_assert

[havhoi92 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87607

            Bug ID: 87607
           Summary: Cannot use fold expression in static_assert
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: havhoi92 at gmail dot com
  Target Milestone: ---

Consider the code snippet:

template<class T, class U>
struct is_same {
    static constexpr bool value = false;
};

template<class T>
struct is_same<T, T> {
    static constexpr bool value = true;
};

template<class T, class... Args>
void f(Args ...args) {
    static_assert((is_same<T, Args>::value) && ...);
}

template<class T, class... Args>
void g(Args ...args) {
    constexpr bool val = ((is_same<T, Args>::value) && ...);
    static_assert(val);
}



Here, g compiles but f does not. I tried several versions on godbolt dot org
after finding the bug on my system, and none of them compiles (Every version
after v7.2).



Compiler output:
<source>: In function 'void f(Args ...)':

<source>:13:45: error: expected ',' before '&&' token

     static_assert((is_same<T, Args>::value) && ...);

                                             ^~

<source>:13:45: error: expected string-literal before '&&' token

<source>:13:45: error: expected ')' before '&&' token

Compiler returned: 1



Flags:
-O2 -std=c++17 -Wall -Wextra -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations



Output of gcc -v:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
7.2.0-1ubuntu1~16.04' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-7
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib
--with-target-system-zlib --enable-objc-gc=auto --enable-multiarch
--disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 7.2.0 (Ubuntu 7.2.0-1ubuntu1~16.04)