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* [Bug c++/98288] New: Accidental equality of classes templated by pointer to local static constant of templated function
@ 2020-12-15 13:58 matthieum.147192 at gmail dot com
2020-12-15 14:38 ` [Bug c++/98288] " matthieum.147192 at gmail dot com
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: matthieum.147192 at gmail dot com @ 2020-12-15 13:58 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/98288] Accidental equality of classes templated by pointer to local static constant of templated function
2020-12-15 13:58 [Bug c++/98288] New: Accidental equality of classes templated by pointer to local static constant of templated function matthieum.147192 at gmail dot com
@ 2020-12-15 14:38 ` matthieum.147192 at gmail dot com
2021-01-04 11:57 ` matthieum.147192 at gmail dot com
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: matthieum.147192 at gmail dot com @ 2020-12-15 14:38 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98288
--- Comment #1 from Matthieu M <matthieum.147192 at gmail dot com> ---
The above program leads to an ICE on gcc (trunk): https://godbolt.org/z/dGe1T6,
from SO user https://stackoverflow.com/users/4832499/passer-by.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/98288] Accidental equality of classes templated by pointer to local static constant of templated function
2020-12-15 13:58 [Bug c++/98288] New: Accidental equality of classes templated by pointer to local static constant of templated function matthieum.147192 at gmail dot com
2020-12-15 14:38 ` [Bug c++/98288] " matthieum.147192 at gmail dot com
@ 2021-01-04 11:57 ` matthieum.147192 at gmail dot com
2021-02-02 9:44 ` matthieum.147192 at gmail dot com
2021-10-02 6:10 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: matthieum.147192 at gmail dot com @ 2021-01-04 11:57 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98288
--- Comment #2 from Matthieu M <matthieum.147192 at gmail dot com> ---
Richard Smith (Clang) agreed that the following program should return 0, and
fixed the behavior in Clang (trunk) in
https://bugs.llvm.org/show_bug.cgi?id=48517.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/98288] Accidental equality of classes templated by pointer to local static constant of templated function
2020-12-15 13:58 [Bug c++/98288] New: Accidental equality of classes templated by pointer to local static constant of templated function matthieum.147192 at gmail dot com
2020-12-15 14:38 ` [Bug c++/98288] " matthieum.147192 at gmail dot com
2021-01-04 11:57 ` matthieum.147192 at gmail dot com
@ 2021-02-02 9:44 ` matthieum.147192 at gmail dot com
2021-10-02 6:10 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: matthieum.147192 at gmail dot com @ 2021-02-02 9:44 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98288
Matthieu M <matthieum.147192 at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Version|7.2.0 |11.0
--- Comment #3 from Matthieu M <matthieum.147192 at gmail dot com> ---
Updated version of compiler.
Also, on gcc trunk this is an ICE:
<source>:18:32: internal compiler error: in discriminator_for_local_entity, at
cp/mangle.c:1949
18 | static constexpr int const I = 2;
| ^
0x1ce5ce9 internal_error(char const*, ...)
???:0
0x6b6f1b fancy_abort(char const*, int, char const*)
???:0
0x812f3b mangle_decl(tree_node*)
???:0
0x135b052 decl_assembler_name(tree_node*)
???:0
0x13da4da make_decl_rtl(tree_node*)
???:0
0x13dd8fd lookup_constant_def(tree_node*)
???:0
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/98288] Accidental equality of classes templated by pointer to local static constant of templated function
2020-12-15 13:58 [Bug c++/98288] New: Accidental equality of classes templated by pointer to local static constant of templated function matthieum.147192 at gmail dot com
` (2 preceding siblings ...)
2021-02-02 9:44 ` matthieum.147192 at gmail dot com
@ 2021-10-02 6:10 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-10-02 6:10 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98288
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |DUPLICATE
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 98930.
*** This bug has been marked as a duplicate of bug 98930 ***
^ permalink raw reply [flat|nested] 5+ messages in thread
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2020-12-15 13:58 [Bug c++/98288] New: Accidental equality of classes templated by pointer to local static constant of templated function matthieum.147192 at gmail dot com
2020-12-15 14:38 ` [Bug c++/98288] " matthieum.147192 at gmail dot com
2021-01-04 11:57 ` matthieum.147192 at gmail dot com
2021-02-02 9:44 ` matthieum.147192 at gmail dot com
2021-10-02 6:10 ` pinskia at gcc dot gnu.org
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