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From: "matthieum.147192 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/98288] New: Accidental equality of classes templated by
 pointer to local static constant of templated function
Date: Tue, 15 Dec 2020 13:58:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98288

            Bug ID: 98288
           Summary: Accidental equality of classes templated by pointer to
                    local static constant of templated function
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: matthieum.147192 at gmail dot com
  Target Milestone: ---

Based on https://stackoverflow.com/q/65306562/147192.

The behavior of GCC (and Clang) is inconsistent (between compile-time and
run-time), however it is unclear to me whether the inconsistency is conform=
ing
with the C++17 standard or not.


The following reduced program is expected to return 0 (invoking g++ with
-std=3Dc++17):

template <typename, typename>
struct is_same { static constexpr bool value =3D false; };

template <typename T>
struct is_same<T, T> { static constexpr bool value =3D true; };

template <typename T, typename U>
static constexpr bool is_same_v =3D is_same<T, U>::value;

using uintptr_t =3D unsigned long long;

template <int const* I>
struct Parameterized { int const* member; };

template <typename T>
auto create() {
    static constexpr int const I =3D 2;

    return Parameterized<&I>{ &I };
}

int main() {
    auto one =3D create<short>();
    auto two =3D create<int>();

    if (is_same_v<decltype(one), decltype(two)>) {
        return reinterpret_cast<uintptr_t>(one.member) =3D=3D
reinterpret_cast<uintptr_t>(two.member) ? 1 : 2;
    }

    return 0;
}

Yet, on all versions of GCC where it compiles (from 7.2 onwards), and for a=
ll
optimization levels (from -O0 to -O3), it returns 2, indicating:

- That `one` and `two` have the same type -- which according to 17.4
[temp.type] should mean that they point to the same object.
- Yet they point to different objects -- there are two instances of
`create<T>()::I`, one for `T =3D short` and one for `T =3D int`.

The assembly listing clearly contains 2 different instances of
`create<T>()::I`.


Notes:

- If `I` is initialized with `=3D sizeof(T)`, instead, then the program ret=
urns 0
as expected.
- Clang (up to 11.0) fails to compile the program with -O0 (the linker fail=
ing
to find a `create()::I` variable), and otherwise produces the same result as
GCC with -O1 to -O3.=