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* [Bug c++/96575] New: std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function
@ 2020-08-12 1:23 hewillk at gmail dot com
2020-08-12 7:33 ` [Bug c++/96575] " hewillk at gmail dot com
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: hewillk at gmail dot com @ 2020-08-12 1:23 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96575
Bug ID: 96575
Summary: std::ranges::sort is not usable as a 'constexpr'
function when saving its return value in lambda
function
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: hewillk at gmail dot com
Target Milestone: ---
(https://godbolt.org/z/W8zx58)
#include <array>
#include <algorithm>
constexpr auto f(auto algo) {
return [=] {
// this one is okay
// algo(std::array{1, 0});
// this one is also okay
// auto it = algo(std::array{0, 1});
auto it = algo(std::array{1, 0});
return 0;
}();
}
int main()
{
static_assert(f(std::ranges::sort) == 0);
}
This fails on gcc 10.2 and trunk with:
<source>:13:23: error: 'constexpr auto f(auto:16) [with auto:16 =
std::ranges::__sort_fn]' called in a constant expression
13 | constexpr auto i = f(std::ranges::sort);
| ~^~~~~~~~~~~~~~~~~~~
<source>:4:16: note: 'constexpr auto f(auto:16) [with auto:16 =
std::ranges::__sort_fn]' is not usable as a 'constexpr' function because:
4 | constexpr auto f(auto algo) {
| ^
<source>:8:8: error: call to non-'constexpr' function
'f<std::ranges::__sort_fn>::<lambda()>'
5 | return [=] {
| ~~~~~
6 | auto it = algo(std::array{1, 0});
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
7 | return 0;
| ~~~~~~~~~
8 | }();
| ~^~
<source>:5:14: note: 'f<std::ranges::__sort_fn>::<lambda()>' is not usable as a
'constexpr' function because:
5 | return [=] {
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/96575] std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function
2020-08-12 1:23 [Bug c++/96575] New: std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function hewillk at gmail dot com
@ 2020-08-12 7:33 ` hewillk at gmail dot com
2020-10-08 3:52 ` ppalka at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: hewillk at gmail dot com @ 2020-08-12 7:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96575
--- Comment #1 from 康桓瑋 <hewillk at gmail dot com> ---
I don't know whether this is same issue or not:
#include <algorithm>
constexpr auto replace = [](auto old_x, auto new_x) {
return [=](auto r) {
return std::ranges::replace(r, old_x, new_x);
};
};
// this one is ok
constexpr auto f1(auto algo) {
[=]() {
algo(std::array{0});
return 0;
}();
return true;
}
// this one fails with non-constant condition
constexpr auto f2(auto algo) {
constexpr auto it = [=]() {
algo(std::array{0});
return 0;
}();
return true;
}
static_assert(f1(replace(0, 1)));
static_assert(f2(replace(0, 1)));
(https://godbolt.org/z/Kaq4j5)
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/96575] std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function
2020-08-12 1:23 [Bug c++/96575] New: std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function hewillk at gmail dot com
2020-08-12 7:33 ` [Bug c++/96575] " hewillk at gmail dot com
@ 2020-10-08 3:52 ` ppalka at gcc dot gnu.org
2020-10-22 11:34 ` cvs-commit at gcc dot gnu.org
2020-10-22 11:51 ` ppalka at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: ppalka at gcc dot gnu.org @ 2020-10-08 3:52 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96575
Patrick Palka <ppalka at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |ASSIGNED
CC| |ppalka at gcc dot gnu.org
Last reconfirmed| |2020-10-08
Assignee|unassigned at gcc dot gnu.org |ppalka at gcc dot gnu.org
--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed. Taking a look.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/96575] std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function
2020-08-12 1:23 [Bug c++/96575] New: std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function hewillk at gmail dot com
2020-08-12 7:33 ` [Bug c++/96575] " hewillk at gmail dot com
2020-10-08 3:52 ` ppalka at gcc dot gnu.org
@ 2020-10-22 11:34 ` cvs-commit at gcc dot gnu.org
2020-10-22 11:51 ` ppalka at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2020-10-22 11:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96575
--- Comment #3 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:46fdced6a9f936ae4d5b42347d7d87f69875683a
commit r11-4230-g46fdced6a9f936ae4d5b42347d7d87f69875683a
Author: Patrick Palka <ppalka@redhat.com>
Date: Thu Oct 22 07:33:58 2020 -0400
c++: constexpr evaluation and bare EMPTY_CLASS_EXPR [PR96575]
In the testcase below, folding of the initializer for 'ret' inside the
instantiated f<lambda>::lambda ends up yielding an initializer for which
potential_constant_expression returns false. This causes finish_function
to mark the lambda as non-constexpr, which ultimately causes us to reject
'f(g)' as a call to a non-constexpr function.
The initializer for 'ret' inside f<lambda>::lambda, prior to folding, is
the CALL_EXPR
<lambda(S)>::operator() (&cb, ({}, <<< Unknown tree: empty_class_expr
>>>;))
where the second argument is a COMPOUND_EXPR whose second operand is an
EMPTY_CLASS_EXPR that was formed by build_class_a. cp_fully_fold_init
is able to only partially fold this initializer: it gets rid of the
side-effectless COMPOUND_EXPR to obtain
<lambda(S)>::operator() (&cb, <<< Unknown tree: empty_class_expr >>>)
as the final initializer for 'ret'. This initializer no longer satifies
potential_constant_expression due to the bare EMPTY_CLASS_EXPR which is
not wrapped in a COMPOUND_EXPR.
(cp_fully_fold_init first tries maybe_constant_value on the original
CALL_EXPR, but constexpr evaluation punts upon seeing
__builtin_is_constant_evaluated, since manifestly_const_eval is false.)
To fix this, it seems we could either make cp_fold preserve the
COMPOUND_EXPR trees produced by build_call_a, or we could improve
the constexpr machinery to treat EMPTY_CLASS_EXPR trees as first-class
citizens. Assuming it's safe to continue folding away these
COMPOUND_EXPRs, the second approach seems cleaner, so this patch
implements the second approach.
gcc/cp/ChangeLog:
PR c++/96575
* constexpr.c (cxx_eval_constant_expression)
<case EMPTY_CLASS_EXPR>: Lower it to a CONSTRUCTOR.
(potential_constant_expression_1) <case COMPOUND_EXPR>: Remove
now-redundant handling of COMPOUND_EXPR with EMPTY_CLASS_EXPR
second operand.
<case EMPTY_CLASS_EXPR>: Return true instead of false.
gcc/testsuite/ChangeLog:
PR c++/96575
* g++.dg/cpp1z/constexpr-96575.C: New test.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/96575] std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function
2020-08-12 1:23 [Bug c++/96575] New: std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function hewillk at gmail dot com
` (2 preceding siblings ...)
2020-10-22 11:34 ` cvs-commit at gcc dot gnu.org
@ 2020-10-22 11:51 ` ppalka at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: ppalka at gcc dot gnu.org @ 2020-10-22 11:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96575
Patrick Palka <ppalka at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |FIXED
Target Milestone|--- |11.0
Status|ASSIGNED |RESOLVED
--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Thanks for the report. The testcase was triggering a bug in the constexpr
evaluator, which should now been fixed.
^ permalink raw reply [flat|nested] 5+ messages in thread
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2020-08-12 1:23 [Bug c++/96575] New: std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function hewillk at gmail dot com
2020-08-12 7:33 ` [Bug c++/96575] " hewillk at gmail dot com
2020-10-08 3:52 ` ppalka at gcc dot gnu.org
2020-10-22 11:34 ` cvs-commit at gcc dot gnu.org
2020-10-22 11:51 ` ppalka at gcc dot gnu.org
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