[Bug c++/97938] New: g++ crash when inferring type of auto parameter pack in lambda capture

[Bug c++/97938] New: g++ crash when inferring type of auto parameter pack in lambda capture

[elel at 3wh dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

            Bug ID: 97938
           Summary: g++ crash when inferring type of auto parameter pack
                    in lambda capture
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: elel at 3wh dot net
  Target Milestone: ---

The following test case:

#include <iostream>
#include <tuple>

template <typename... Args>
int print_args(Args&&... args) {
  std::cout << (args << ...);
  return sizeof...(args);
}

template <typename T1, typename T2>
auto fwd(const T1& t1, const T2& t2) {
  return std::apply([&t2] (auto&&... ts1) {
    return std::apply([...ts1 = std::forward<decltype(ts1)>(ts1)] (auto&&...
ts2) {
      return print_args(ts1..., std::forward<decltype(ts2)>(ts2)...);
    }, t2);
  }, t1);
}

int main() {
  auto t1 = std::make_tuple(int{1}, float{2});
  auto t2 = std::make_tuple(float{3}, int{4});
  return fwd(t1, t2);
}

Compiled as:
g++ -std=c++20 -g -O2 gcc_crash.cpp

Crashes g++ with the following message:
gcc_crash.cpp: In instantiation of ‘fwd<std::tuple<int, float>,
std::tuple<float, int> >::<lambda(auto:11&& ...)> [with auto:11 = {const int&,
const float&}]’:
/usr/include/c++/10.2.0/type_traits:2506:26:   required by substitution of
‘template<class _Fn, class ... _Args> static std::__result_of_success<decltype
(declval<_Fn>()((declval<_Args>)()...)), std::__invoke_other>
std::__result_of_other_impl::_S_test(int) [with _Fn = fwd<std::tuple<int,
float>, std::tuple<float, int> >::<lambda(auto:11&& ...)>; _Args = {const int&,
const float&}]’
/usr/include/c++/10.2.0/type_traits:2517:55:   required from ‘struct
std::__result_of_impl<false, false, fwd<std::tuple<int, float>,
std::tuple<float, int> >::<lambda(auto:11&& ...)>, const int&, const float&>’
/usr/include/c++/10.2.0/type_traits:138:12:   recursively required by
substitution of ‘template<class _Result, class _Ret> struct
std::__is_invocable_impl<_Result, _Ret, true, std::__void_t<typename
_CTp::type> > [with _Result = std::__invoke_result<fwd<std::tuple<int, float>,
std::tuple<float, int> >::<lambda(auto:11&& ...)>, const int&, const float&>;
_Ret = void]’
/usr/include/c++/10.2.0/type_traits:138:12:   required from ‘struct
std::__and_<std::__is_invocable_impl<std::__invoke_result<fwd<std::tuple<int,
float>, std::tuple<float, int> >::<lambda(auto:11&& ...)>, const int&, const
float&>, void, true, void>,
std::__call_is_nothrow<std::__invoke_result<fwd<std::tuple<int, float>,
std::tuple<float, int> >::<lambda(auto:11&& ...)>, const int&, const float&>,
fwd<std::tuple<int, float>, std::tuple<float, int> >::<lambda(auto:11&& ...)>,
const int&, const float&> >’
/usr/include/c++/10.2.0/type_traits:2979:12:   required from ‘struct
std::is_nothrow_invocable<fwd<std::tuple<int, float>, std::tuple<float, int>
>::<lambda(auto:11&& ...)>, const int&, const float&>’
/usr/include/c++/10.2.0/tuple:1715:37:   required from ‘constexpr const bool
std::__unpack_std_tuple<template<class _Fn, class ... _ArgTypes> struct
std::is_nothrow_invocable, fwd<std::tuple<int, float>, std::tuple<float, int>
>::<lambda(auto:11&& ...)>, const std::tuple<int, float>&>’
/usr/include/c++/10.2.0/tuple:1730:14:   required from ‘constexpr
decltype(auto) std::apply(_Fn&&, _Tuple&&) [with _Fn = fwd<std::tuple<int,
float>, std::tuple<float, int> >::<lambda(auto:11&& ...)>; _Tuple = const
std::tuple<int, float>&]’
gcc_crash.cpp:12:20:   required from ‘auto fwd(const T1&, const T2&) [with T1 =
std::tuple<int, float>; T2 = std::tuple<float, int>]’
gcc_crash.cpp:22:20:   required from here
gcc_crash.cpp:13:23: internal compiler error: Segmentation fault
   13 |     return std::apply([...ts1 = std::forward<decltype(ts1)>(ts1)]
(auto&&... ts2) {
      |                      
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   14 |       return print_args(ts1..., std::forward<decltype(ts2)>(ts2)...);
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   15 |     }, t2);
      |     ~                  
Please submit a full bug report,
with preprocessed source if appropriate.

In other scenarios (not the minimal test case above) the same bug triggers the
message:
internal compiler error: in cxx_incomplete_type_diagnostic, at cp/typeck2.c:584

[Bug c++/97938] g++ crash when inferring type of auto parameter pack in lambda capture

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |mpolacek at gcc dot gnu.org
   Last reconfirmed|                            |2020-11-23

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.

[Bug c++/97938] 9/10/11 Regression] g++ crash when inferring type of auto parameter pack in lambda capture

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|g++ crash when inferring    |9/10/11 Regression]  g++
                   |type of auto parameter pack |crash when inferring type
                   |in lambda capture           |of auto parameter pack in
                   |                            |lambda capture
   Target Milestone|---                         |9.4

[Bug c++/97938] [9/10/11 Regression] g++ crash when inferring type of auto parameter pack in lambda capture

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|9/10/11 Regression]  g++    |[9/10/11 Regression]  g++
                   |crash when inferring type   |crash when inferring type
                   |of auto parameter pack in   |of auto parameter pack in
                   |lambda capture              |lambda capture
           Keywords|                            |ice-on-valid-code
           Priority|P3                          |P2

[Bug c++/97938] [9/10/11 Regression] g++ crash when inferring type of auto parameter pack in lambda capture

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

Jason Merrill <jason at gcc dot gnu.org> changed:

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

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
Removed library dependency:

template <class F, class... Args>
auto apply(F&& f, Args&&... args) { return f(args...); }

template <class T> T&& forward(T&& t);

template <typename... Args>
int print_args(Args&&... args) {
  return sizeof...(args);
}

template <typename T1, typename T2>
auto fwd(const T1& t1, const T2& t2) {
  return ::apply([&t2] (auto&&... ts1) {
    return ::apply([...ts1 = forward<decltype(ts1)>(ts1)] (auto&&... ts2) {
      return print_args(ts1..., forward<decltype(ts2)>(ts2)...);
    }, t2);
  }, t1);
}

int main() {
  auto t1 = 1;
  auto t2 = 2;
  return fwd(t1, t2);
}

[Bug c++/97938] [9/10/11 Regression] g++ crash when inferring type of auto parameter pack in lambda capture

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
Reduced more:

template <typename... Args>
int sink(Args&&... args) { return 2; }

template <typename T1>
auto fwd(const T1& t1) {
  return
    [] (auto&&... ts1) {
      return
        [...ts1 = ts1] () {
          return sink(ts1...);
        }();
    }();
}

int main() {
  return fwd(1);
}

[Bug c++/97938] [9/10/11 Regression] g++ crash when inferring type of auto parameter pack in lambda capture

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:2a26351b598242c2fbce95d2a0baacce0084aec6

commit r11-7961-g2a26351b598242c2fbce95d2a0baacce0084aec6
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Apr 2 11:05:46 2021 -0400

    c++: lambda pack init-capture within generic lambda

    We represent the type of a pack init-capture as auto... with packs from the
    initializer stuck into PACK_EXPANSION_PARAMETER_PACKS so that expanding it
    produces the right number of elements.  But when partially instantiating
the
    auto..., we were changing PACK_EXPANSION_PARAMETER_PACKS to refer to only
    the auto itself.  Fixed thus.

    gcc/cp/ChangeLog:

            PR c++/97938
            * cp-tree.h (PACK_EXPANSION_AUTO_P): New.
            * lambda.c (add_capture): Set it.
            * pt.c (tsubst_pack_expansion): Handle it.

    gcc/testsuite/ChangeLog:

            PR c++/97938
            * g++.dg/cpp2a/lambda-pack-init6.C: New test.

[Bug c++/97938] [9/10 Regression] g++ crash when inferring type of auto parameter pack in lambda capture

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[9/10/11 Regression]  g++   |[9/10 Regression]  g++
                   |crash when inferring type   |crash when inferring type
                   |of auto parameter pack in   |of auto parameter pack in
                   |lambda capture              |lambda capture

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for GCC 11 so far.  Note that the original testcase has a syntax error in
print_args; the fold-expression line should be

  (std::cout << ... << args);

[Bug c++/97938] [9/10 Regression] g++ crash when inferring type of auto parameter pack in lambda capture

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

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

https://gcc.gnu.org/g:5e7801808d2e9e99dc2931462c69a80c6f0e89cc

commit r10-9840-g5e7801808d2e9e99dc2931462c69a80c6f0e89cc
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Apr 2 11:05:46 2021 -0400

    c++: lambda pack init-capture within generic lambda

    We represent the type of a pack init-capture as auto... with packs from the
    initializer stuck into PACK_EXPANSION_PARAMETER_PACKS so that expanding it
    produces the right number of elements.  But when partially instantiating
the
    auto..., we were changing PACK_EXPANSION_PARAMETER_PACKS to refer to only
    the auto itself.  Fixed thus.

    gcc/cp/ChangeLog:

            PR c++/97938
            * cp-tree.h (PACK_EXPANSION_AUTO_P): New.
            * lambda.c (add_capture): Set it.
            * pt.c (tsubst_pack_expansion): Handle it.

    gcc/testsuite/ChangeLog:

            PR c++/97938
            * g++.dg/cpp2a/lambda-pack-init6.C: New test.

[Bug c++/97938] [9/10 Regression] g++ crash when inferring type of auto parameter pack in lambda capture

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

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

https://gcc.gnu.org/g:7168fb621b4442936d2a9429389a65f15043e5e4

commit r9-9549-g7168fb621b4442936d2a9429389a65f15043e5e4
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Apr 2 11:05:46 2021 -0400

    c++: lambda pack init-capture within generic lambda

    We represent the type of a pack init-capture as auto... with packs from the
    initializer stuck into PACK_EXPANSION_PARAMETER_PACKS so that expanding it
    produces the right number of elements.  But when partially instantiating
the
    auto..., we were changing PACK_EXPANSION_PARAMETER_PACKS to refer to only
    the auto itself.  Fixed thus.

    gcc/cp/ChangeLog:

            PR c++/97938
            * cp-tree.h (PACK_EXPANSION_AUTO_P): New.
            * lambda.c (add_capture): Set it.
            * pt.c (tsubst_pack_expansion): Handle it.

    gcc/testsuite/ChangeLog:

            PR c++/97938
            * g++.dg/cpp2a/lambda-pack-init6.C: New test.

[Bug c++/97938] [9/10 Regression] g++ crash when inferring type of auto parameter pack in lambda capture

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

Jason Merrill <jason at gcc dot gnu.org> changed:

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

--- Comment #8 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for 9.4/10.4/11.

[Bug c++/97938] [9/10 Regression] g++ crash when inferring type of auto parameter pack in lambda capture

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Christophe Lyon
<clyon@gcc.gnu.org>:

https://gcc.gnu.org/g:ba19b5543d31a76f494e76c32a548d70ac4d3cc1

commit r9-9553-gba19b5543d31a76f494e76c32a548d70ac4d3cc1
Author: Christophe Lyon <christophe.lyon@linaro.org>
Date:   Mon May 24 07:35:44 2021 +0000

    c++: lambda pack init-capture within generic lambda

    In gcc-9-branch, the testsuite does not support the c++20 target
    selector: use c++2a instead, to avoid Dejagnu errors when running the
    testsuite.

    2021-05-24  Christophe Lyon  <christophe.lyon@linaro.org>

            PR c++/97938
            gcc/testsuite/
            * g++.dg/cpp2a/lambda-pack-init6.C: Use c++2a target selector.

[Bug c++/97938] [9/10 Regression] g++ crash when inferring type of auto parameter pack in lambda capture

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97938

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.4                         |10.4

--- Comment #10 from Jason Merrill <jason at gcc dot gnu.org> ---
Not actually fixed on 9 branch.