https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98288
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 conforming
with the C++17 standard or not.
The following reduced program is expected to return 0 (invoking g++ with
-std=c++17):
template <typename, typename>
struct is_same { static constexpr bool value = false; };
template <typename T>
struct is_same<T, T> { static constexpr bool value = true; };
template <typename T, typename U>
static constexpr bool is_same_v = is_same<T, U>::value;
using uintptr_t = unsigned long long;
template <int const* I>
struct Parameterized { int const* member; };
template <typename T>
auto create() {
static constexpr int const I = 2;
return Parameterized<&I>{ &I };
}
int main() {
auto one = create<short>();
auto two = create<int>();
if (is_same_v<decltype(one), decltype(two)>) {
return reinterpret_cast<uintptr_t>(one.member) ==
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 all
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 = short` and one for `T = int`.
The assembly listing clearly contains 2 different instances of
`create<T>()::I`.
Notes:
- If `I` is initialized with `= sizeof(T)`, instead, then the program returns 0
as expected.
- Clang (up to 11.0) fails to compile the program with -O0 (the linker failing
to find a `create()::I` variable), and otherwise produces the same result as
GCC with -O1 to -O3.
