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From: "cooky.ykooc922 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/105520] New: Ignores constraint in auto declaration with
 braced-init-list
Date: Sun, 08 May 2022 01:55:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105520

            Bug ID: 105520
           Summary: Ignores constraint in auto declaration with
                    braced-init-list
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cooky.ykooc922 at gmail dot com
  Target Milestone: ---

>>From the code snippet below:

#include <concepts>
#include <initializer_list>

template <typename T>
concept always_true =3D true;

template <typename T>
concept always_false =3D false;

template <typename>
inline constexpr bool is_init_list_v =3D false;

template <typename T>
inline constexpr bool is_init_list_v<std::initializer_list<T>> =3D true;

template <typename T>
concept not_init_list =3D !is_init_list_v<std::remove_cvref_t<T>>;

int main() {
    always_true auto a =3D 10; // ok
    always_true auto b =3D {1, 2, 3}; // ok

    // always_false auto c =3D 10; // error as expected
    // always_false auto d =3D {1, 2, 3}; // error as expected

    not_init_list auto e =3D 10; // ok
    not_init_list auto f =3D {1, 2, 3}; // no error but unexpected

    // error as expected below:
    // not_init_list auto g =3D std::initializer_list{1, 2, 3};=20
}

GCC accepts invalid code from `not_init_list auto f =3D {1, 2, 3}`, while C=
lang
rejects it as expected showing:

<source>:27:5: error: deduced type 'std::initializer_list<int>' does not
satisfy 'not_init_list'
    not_init_list auto f =3D {1, 2, 3}; // no error but unexpected
    ^~~~~~~~~~~~~~~~~~
<source>:17:25: note: because
'!is_init_list_v<std::remove_cvref_t<initializer_list<int> > >' evaluated to
false
concept not_init_list =3D !is_init_list_v<std::remove_cvref_t<T>>;
                        ^
compiler flag: -std=3Dc++20
compiler explorer link: https://godbolt.org/z/G569rxb8f=