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From: "vince.rev at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/95036] New: ICE with variadic type/nttp template templates
Date: Sun, 10 May 2020 13:29:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95036

            Bug ID: 95036
           Summary: ICE with variadic type/nttp template templates
           Product: gcc
           Version: 10.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vince.rev at gmail dot com
  Target Milestone: ---

Created attachment 48497
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D48497&action=3Dedit
ICE with variadic type/nttp template templates

The following code in std=3Dc++17 (I couldn't make it smaller because I don=
't
know where the bug is coming from) compiles correctly with gcc 7 and 8, but
fails with gcc 9 and 10. Link to godbolt: https://godbolt.org/z/x_66lB

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
#include <array>
#include <utility>
#include <type_traits>

template <template <class, auto...> class Template>
struct base {
    template <template <class, auto...> class, class =3D void>
    struct is_same_template: std::false_type {};
    template <class Dummy>
    struct is_same_template<Template, Dummy>: std::true_type {};
    template <
        template <class, auto...> class X,
        class =3D std::enable_if_t<is_same_template<X>::value>
    >
    constexpr void function() const noexcept {};
};

template <template <class, auto...> class... Templates>
struct derived: base<Templates>... {
    using base<Templates>::function...;
};

int main(int, char**) {
    derived<std::array, std::integer_sequence> x;
    x.function<std::array>();
}
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

The error message is :
---------------------------------------------------------------------------=
-----
gcc-ice-2020-05-10.cpp: In substitution of 'template<template<class, auto
...<anonymous> > class X, class> constexpr void
base<std::integer_sequence>::function<X, <template-parameter-1-2> >() const
[with X =3D std::array; <template-parameter-1-2> =3D <missing>]':
gcc-ice-2020-05-10.cpp:25:28:   required from here
gcc-ice-2020-05-10.cpp:10:12: internal compiler error: in tsubst, at
cp/pt.c:14592
   10 |     struct is_same_template<Template, Dummy>: std::true_type {};
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Please submit a full bug report,
with preprocessed source if appropriate.
See <file:///usr/share/doc/gcc-9/README.Bugs> for instructions.
---------------------------------------------------------------------------=
-----=