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

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

[csaba_22 at yahoo dot co.uk]

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

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

[jakub at gcc dot gnu.org]

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

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
cvise reduced into:
template <int a> struct b { static constexpr int value = a; };
template <typename c> c e(long);
template <typename c> auto d() -> decltype(e<c>(0));
template <typename> struct tuple_size;
template <unsigned long, typename> struct tuple_element;
template <typename...> struct l { template <int> void get(); };
l<int, int, int, void> f;
template <typename... g> struct tuple_size<l<g...>> : b<sizeof... (g)> {};
template <unsigned long m, class... g> struct tuple_element<m, l<g...>> {
  using type = decltype(d<l<>>().get<m>());
};
auto [h, i, j, k] = f;

Another testcase that ICEs the same way:
namespace std {
  template<typename T> struct tuple_size;
  template<int, typename> struct tuple_element;
}
struct A {
  int i;
  template <int I> void get() { }
};
template<> struct std::tuple_size<A> { static const int value = 2; };
template<int I> struct std::tuple_element<I,A> { using type = void; };
A a = { 42 };
auto [ x, y ] = a;

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

[mpolacek at gcc dot gnu.org]

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

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-03-18
             Status|UNCONFIRMED                 |NEW
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Started with r242377.

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

[jakub at gcc dot gnu.org]

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

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |jakub at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 50425
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50425&action=edit
gcc11-pr99650.patch

Untested fix.

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

[csaba_22 at yahoo dot co.uk]

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

--- Comment #4 from Csaba Ráduly <csaba_22 at yahoo dot co.uk> ---
Is this a dup of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79372 ? 

https://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg522821.html

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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:3e07e7a6a7f34f0ec2f1a3e50ebc52b77de11a30

commit r11-7776-g3e07e7a6a7f34f0ec2f1a3e50ebc52b77de11a30
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Mar 23 10:23:42 2021 +0100

    c++: Diagnose references to void in structured bindings [PR99650]

    We ICE on the following testcase, because std::tuple_element<...,...>::type
    is void and for structured bindings we therefore need to create
    void & or void && which is invalid.  We created such REFERENCE_TYPE and
    later ICEd in the middle-end.
    The following patch fixes it by diagnosing that.

    2021-03-23  Jakub Jelinek  <jakub@redhat.com>

            PR c++/99650
            * decl.c (cp_finish_decomp): Diagnose void initializers when
            using tuple_element and get.

            * g++.dg/cpp1z/decomp55.C: New test.

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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:d5e379e3fe19362442b5d0ac608fb8ddf67fecd3

commit r10-9621-gd5e379e3fe19362442b5d0ac608fb8ddf67fecd3
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Mar 23 10:23:42 2021 +0100

    c++: Diagnose references to void in structured bindings [PR99650]

    We ICE on the following testcase, because std::tuple_element<...,...>::type
    is void and for structured bindings we therefore need to create
    void & or void && which is invalid.  We created such REFERENCE_TYPE and
    later ICEd in the middle-end.
    The following patch fixes it by diagnosing that.

    2021-03-23  Jakub Jelinek  <jakub@redhat.com>

            PR c++/99650
            * decl.c (cp_finish_decomp): Diagnose void initializers when
            using tuple_element and get.

            * g++.dg/cpp1z/decomp55.C: New test.

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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:dd320787b4eb11521e3ae3f9aa9504b31ee08c36

commit r9-9436-gdd320787b4eb11521e3ae3f9aa9504b31ee08c36
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Mar 23 10:23:42 2021 +0100

    c++: Diagnose references to void in structured bindings [PR99650]

    We ICE on the following testcase, because std::tuple_element<...,...>::type
    is void and for structured bindings we therefore need to create
    void & or void && which is invalid.  We created such REFERENCE_TYPE and
    later ICEd in the middle-end.
    The following patch fixes it by diagnosing that.

    2021-03-23  Jakub Jelinek  <jakub@redhat.com>

            PR c++/99650
            * decl.c (cp_finish_decomp): Diagnose void initializers when
            using tuple_element and get.

            * g++.dg/cpp1z/decomp55.C: New test.

    (cherry picked from commit d5e379e3fe19362442b5d0ac608fb8ddf67fecd3)

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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-8 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:6132dc70413163b96955a3ebca33c849eb2eb88e

commit r8-10899-g6132dc70413163b96955a3ebca33c849eb2eb88e
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Mar 23 10:23:42 2021 +0100

    c++: Diagnose references to void in structured bindings [PR99650]

    We ICE on the following testcase, because std::tuple_element<...,...>::type
    is void and for structured bindings we therefore need to create
    void & or void && which is invalid.  We created such REFERENCE_TYPE and
    later ICEd in the middle-end.
    The following patch fixes it by diagnosing that.

    2021-03-23  Jakub Jelinek  <jakub@redhat.com>

            PR c++/99650
            * decl.c (cp_finish_decomp): Diagnose void initializers when
            using tuple_element and get.

            * g++.dg/cpp1z/decomp55.C: New test.

    (cherry picked from commit d5e379e3fe19362442b5d0ac608fb8ddf67fecd3)

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

[jakub at gcc dot gnu.org]

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

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed.

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

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |8.5