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* [Bug c++/99130] New: Variable template in unevaluated context is wrongly implicitly instantiated
@ 2021-02-17 5:52 xskxzr at gmail dot com
2021-08-17 9:37 ` [Bug c++/99130] " pinskia at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: xskxzr at gmail dot com @ 2021-02-17 5:52 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99130
Bug ID: 99130
Summary: Variable template in unevaluated context is wrongly
implicitly instantiated
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: xskxzr at gmail dot com
Target Milestone: ---
Motivated by the StackOverflow post:
https://stackoverflow.com/questions/66214214/different-sfinae-behavior-of-stdtuple-size-v-on-different-compilers
The following code does not compile:
#include <type_traits>
#include <iostream>
template<typename T> constexpr int f() { return T::value; }
template<typename T> constexpr int z = T::value;
template<typename T, typename = int> struct G : std::false_type {};
template<typename T> struct G<T, decltype(z<T>)> : std::true_type {}; //
error
// template<typename T> struct G<T, decltype(f<T>())> : std::true_type {};
// ok
int main() {
std::cout << G<int>::value;
}
Here the definition of z<T> is not required, so its definition shouldn't be
implicitly instantiated.
Note if we replace z<T> by f<T>(), the code compiles successfully.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/99130] Variable template in unevaluated context is wrongly implicitly instantiated
2021-02-17 5:52 [Bug c++/99130] New: Variable template in unevaluated context is wrongly implicitly instantiated xskxzr at gmail dot com
@ 2021-08-17 9:37 ` pinskia at gcc dot gnu.org
2021-08-17 9:51 ` [Bug c++/99130] variable template in unevaluated context is instantiated when should not be pinskia at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-08-17 9:37 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99130
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Interesting only clang accepts the code, ICC and MSVC reject it for the same
reason as GCC.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/99130] variable template in unevaluated context is instantiated when should not be
2021-02-17 5:52 [Bug c++/99130] New: Variable template in unevaluated context is wrongly implicitly instantiated xskxzr at gmail dot com
2021-08-17 9:37 ` [Bug c++/99130] " pinskia at gcc dot gnu.org
@ 2021-08-17 9:51 ` pinskia at gcc dot gnu.org
2021-08-17 10:02 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-08-17 9:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99130
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|Variable template in |variable template in
|unevaluated context is |unevaluated context is
|wrongly implicitly |instantiated when should
|instantiated |not be
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced testcase:
template<typename T> constexpr int z = T::value;
int main() {
decltype(z<int>) a = 1;
}
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/99130] variable template in unevaluated context is instantiated when should not be
2021-02-17 5:52 [Bug c++/99130] New: Variable template in unevaluated context is wrongly implicitly instantiated xskxzr at gmail dot com
2021-08-17 9:37 ` [Bug c++/99130] " pinskia at gcc dot gnu.org
2021-08-17 9:51 ` [Bug c++/99130] variable template in unevaluated context is instantiated when should not be pinskia at gcc dot gnu.org
@ 2021-08-17 10:02 ` pinskia at gcc dot gnu.org
2022-09-08 13:45 ` cvs-commit at gcc dot gnu.org
2022-09-08 13:48 ` ppalka at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-08-17 10:02 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99130
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2021-08-17
Ever confirmed|0 |1
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Here is another example:
template<typename T> constexpr int z = T::value;
int main() {
return sizeof(z<int>) != sizeof(int);
}
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/99130] variable template in unevaluated context is instantiated when should not be
2021-02-17 5:52 [Bug c++/99130] New: Variable template in unevaluated context is wrongly implicitly instantiated xskxzr at gmail dot com
` (2 preceding siblings ...)
2021-08-17 10:02 ` pinskia at gcc dot gnu.org
@ 2022-09-08 13:45 ` cvs-commit at gcc dot gnu.org
2022-09-08 13:48 ` ppalka at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2022-09-08 13:45 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99130
--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:
https://gcc.gnu.org/g:4db3cb781c355341fa041e6b5bbbfc495c6a0fdb
commit r13-2540-g4db3cb781c355341fa041e6b5bbbfc495c6a0fdb
Author: Patrick Palka <ppalka@redhat.com>
Date: Thu Sep 8 09:45:45 2022 -0400
c++: unnecessary instantiation of constexpr var [PR99130]
Here the use of 'value' from within an unevaluated context causes us
to overeagerly instantiate it, via maybe_instantiate_decl called from
mark_used, despite the use occurring in a context that doesn't require
a definition.
This seems to only affect constexpr variable specializations, though
we used to have the same issue for constexpr function specializations
until r6-1309-g81371eff9bc7ef made us delay their instantiation until
necessary during constexpr evaluation.
This patch expands upon the r6-1309 fix to make mark_used avoid
unnecessarily instantiating constexpr variable specializations too,
by pulling out from maybe_instantiate_decl the condition
(decl_maybe_constant_var_p (decl)
|| (TREE_CODE (decl) == FUNCTION_DECL
&& DECL_OMP_DECLARE_REDUCTION_P (decl))
|| undeduced_auto_decl (decl))
into each of its three callers (including mark_used), removing the
problematic first test from mark_used, and simplifying accordingly.
The net result is that only mark_used is changed because the other two
callers, resolve_address_of_overloaded_function and decl_constant_var_p,
already guard the call appropriately. (This relaxation of mark_used
seems to be safe because during constexpr evaluation we already take
care to instantiate a constexpr variable as necessary via
decl_constant_value etc).
PR c++/99130
gcc/cp/ChangeLog:
* decl2.cc (maybe_instantiate_decl): Adjust function comment.
Check VAR_OR_FUNCTION_DECL_P. Pull out the disjunction into ...
(mark_used): ... here, removing the decl_maybe_constant_var_p
part of it.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/constexpr-decltype5.C: New test.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/99130] variable template in unevaluated context is instantiated when should not be
2021-02-17 5:52 [Bug c++/99130] New: Variable template in unevaluated context is wrongly implicitly instantiated xskxzr at gmail dot com
` (3 preceding siblings ...)
2022-09-08 13:45 ` cvs-commit at gcc dot gnu.org
@ 2022-09-08 13:48 ` ppalka at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: ppalka at gcc dot gnu.org @ 2022-09-08 13:48 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99130
Patrick Palka <ppalka at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |FIXED
Assignee|unassigned at gcc dot gnu.org |ppalka at gcc dot gnu.org
Status|NEW |RESOLVED
Target Milestone|--- |13.0
CC| |ppalka at gcc dot gnu.org
--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 13, thanks for the bug report.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2022-09-08 13:48 UTC | newest]
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2021-02-17 5:52 [Bug c++/99130] New: Variable template in unevaluated context is wrongly implicitly instantiated xskxzr at gmail dot com
2021-08-17 9:37 ` [Bug c++/99130] " pinskia at gcc dot gnu.org
2021-08-17 9:51 ` [Bug c++/99130] variable template in unevaluated context is instantiated when should not be pinskia at gcc dot gnu.org
2021-08-17 10:02 ` pinskia at gcc dot gnu.org
2022-09-08 13:45 ` cvs-commit at gcc dot gnu.org
2022-09-08 13:48 ` ppalka at gcc dot gnu.org
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