[Bug c++/99650] New: ICE when trying to form reference to void in structured binding

["csaba_22 at yahoo dot co.uk" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99650

            Bug ID: 99650
           Summary: ICE when trying to form reference to void in
                    structured binding
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: csaba_22 at yahoo dot co.uk
  Target Milestone: ---

Created attachment 50423
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50423&action=edit
gzipped preprocessor output

The following code leads to an internal compiler error:

#include <stdint.h>
#include <tuple>
#include <string>

struct Cow{};
struct Dog{};
struct Horse{}; // of course

template <typename ... Ts>
struct meow{

    template<size_t Idx>
    decltype(auto)
    get()
    {
        if      constexpr (Idx == 0) { return Cow{}; }
        else if constexpr (Idx == 1) { return Dog{}; }
        else if constexpr (Idx == 2) { return Horse{}; }
    }
};

template <typename ... Ts>
meow<Ts...> kitty()
{
    return meow<Ts...>{};
}

namespace std
{
    template<typename ... Ts>
    struct tuple_size< meow<Ts...> >
    : std::integral_constant<std::size_t, sizeof...(Ts)> {};

    template< std::size_t Idx, class... Ts >
    struct tuple_element< Idx, meow<Ts...> >
    {
        using type = decltype(  std::declval<  meow<Ts...>  >().template
get<Idx>()  );
    };
}

int main(int argc, char const *argv[])
{
    auto && [ moo, woof, neigh, vv ] = kitty<Cow, Dog, Horse, void>();
}



$ g++ -v g++ -g -std=gnu++17 -fdiagnostics-color   -g 
tuple-protocol-template.cpp -o tuple-protocol-template
Using built-in specs.
COLLECT_GCC=g++
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
7.5.0-3ubuntu1~18.04' --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 --enable-bootstrap --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 --enable-default-pie
--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 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)
tuple-protocol-template.cpp: In function ‘int main(int, const char**)’:
tuple-protocol-template.cpp:43:69: internal compiler error: in
build_target_expr_with_type, at cp/tree.c:722
     auto && [ moo, woof, neigh, vv ] = kitty<Cow, Dog, Horse, void>();
                                                                     ^

Also fails with:
* gcc version 11.0.1 20210317 (experimental) (Compiler-Explorer-Build)
* GNU C++17 (Compiler-Explorer-Build) version 10.2.0 (x86_64-linux-gnu)
        compiled by GNU C version 7.5.0, GMP version 6.1.0, MPFR version 3.1.4,
MPC version 1.0.3, isl version isl-0.18-GMP
* GNU C++17 (Compiler-Explorer-Build) version 9.3.0 (x86_64-linux-gnu)
        compiled by GNU C version 7.5.0, GMP version 6.1.0, MPFR version 3.1.4,
MPC version 1.0.3, isl version isl-0.18-GMP
* GNU C++17 (Compiler-Explorer-Build) version 8.3.0 (x86_64-linux-gnu)
        compiled by GNU C version 7.5.0, GMP version 6.1.0, MPFR version 3.1.4,
MPC version 1.0.3, isl version isl-0.18-GMP
* GNU C++14 (GCC-Explorer-Build) version 7.1.0 (x86_64-linux-gnu)
        compiled by GNU C version 5.4.0 20160609, GMP version 6.1.0, MPFR
version 3.1.4, MPC version 1.0.3, isl version isl-0.16.1-GMP


clang 11.0.1 on godbolt reports:

<source>:43:33: error: cannot form a reference to 'void'
    auto && [ moo, woof, neigh, vv ] = kitty<Cow, Dog, Horse, void>();
                                ^
<source>:43:33: note: in implicit initialization of binding declaration 'vv'

https://godbolt.org/z/e18Ts6