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* [Bug c++/101557] New: the value of '<temporary>' is not usable in a constant expression
@ 2021-07-21 15:13 federico.kircheis at gmail dot com
2021-07-21 15:24 ` [Bug c++/101557] " jakub at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: federico.kircheis at gmail dot com @ 2021-07-21 15:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101557
Bug ID: 101557
Summary: the value of '<temporary>' is not usable in a constant
expression
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: federico.kircheis at gmail dot com
Target Milestone: ---
Consider following snippet
----
struct node {
const char* d;
const node& left;
const node& right;
};
constexpr const node Null = node{"",Null,Null};
constexpr const node a
{
"a",
node
{
"b",
node{"e",Null,Null},
node{"d",Null,Null},
},
node{"c",Null,Null},
};
constexpr int mysize(const node& n) noexcept{
return (&n == &Null) ? 0 : 1 + mysize(n.left) + mysize(n.right);
}
constexpr auto size0 = mysize(Null);
static_assert(size0 == 0, "");
constexpr auto size1 = mysize(node{"c", Null, Null});
static_assert(size1 == 1, "");
constexpr auto size2 = mysize(node
{
"b",
node{"e",Null,Null},
node{"d",Null,Null},
});
static_assert(size2 == 3, "");
// this line does not compile
constexpr auto size3 = mysize(a);
static_assert(size3 == 5, "");
----
The expression `constexpr auto size3 = mysize(a);` does not compile with the
error message
----
<source>:41:30: in 'constexpr' expansion of 'mysize(a)'
<source>:25:42: in 'constexpr' expansion of 'mysize((* & n.node::left))'
<source>:41:32: error: the value of '<temporary>' is not usable in a constant
expression
41 | constexpr auto size3 = mysize(a);
| ^
<source>:22:1: note: '<temporary>' was not declared 'constexpr'
22 | };
| ^
Compiler returned: 1
----
A similar effect can be achieved with
----
constexpr node b = a.left;
----
with following error
----
<source>:45:23: error: the value of '<temporary>' is not usable in a constant
expression
45 | constexpr node b2 = a.left;
| ^~~~
<source>:22:1: note: '<temporary>' was not declared 'constexpr'
22 | };
| ^
Compiler returned: 1
----
even if
----
constexpr node b = a;
----
compiles without warnings.
Generally accessing subobjects should not be an issue, for example
----
struct sub{};
struct node2{
const sub& s;
};
constexpr const node2 n2{
sub{}
};
constexpr auto s = n2.s;
----
also compiles without issues
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/101557] the value of '<temporary>' is not usable in a constant expression
2021-07-21 15:13 [Bug c++/101557] New: the value of '<temporary>' is not usable in a constant expression federico.kircheis at gmail dot com
@ 2021-07-21 15:24 ` jakub at gcc dot gnu.org
2021-07-21 16:03 ` federico.kircheis at gmail dot com
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: jakub at gcc dot gnu.org @ 2021-07-21 15:24 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101557
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
clang++ rejects it similarly.
pr101557.C:40:16: error: constexpr variable 'size3' must be initialized by a
constant expression
constexpr auto size3 = mysize(a);
^ ~~~~~~~~~
pr101557.C:22:45: note: read of temporary is not allowed in a constant
expression outside the expression that created the temporary
Wonder if this isn't related to the recently posted PR100976 patch.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/101557] the value of '<temporary>' is not usable in a constant expression
2021-07-21 15:13 [Bug c++/101557] New: the value of '<temporary>' is not usable in a constant expression federico.kircheis at gmail dot com
2021-07-21 15:24 ` [Bug c++/101557] " jakub at gcc dot gnu.org
@ 2021-07-21 16:03 ` federico.kircheis at gmail dot com
2021-07-21 16:07 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: federico.kircheis at gmail dot com @ 2021-07-21 16:03 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101557
--- Comment #2 from Federico Kircheis <federico.kircheis at gmail dot com> ---
clang does not reject it:
https://godbolt.org/z/8Mq1e3o3j
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/101557] the value of '<temporary>' is not usable in a constant expression
2021-07-21 15:13 [Bug c++/101557] New: the value of '<temporary>' is not usable in a constant expression federico.kircheis at gmail dot com
2021-07-21 15:24 ` [Bug c++/101557] " jakub at gcc dot gnu.org
2021-07-21 16:03 ` federico.kircheis at gmail dot com
@ 2021-07-21 16:07 ` pinskia at gcc dot gnu.org
2021-07-21 16:15 ` federico.kircheis at gmail dot com
2023-04-16 9:31 ` federico at kircheis dot it
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-07-21 16:07 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101557
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Federico Kircheis from comment #2)
> clang does not reject it:
>
> https://godbolt.org/z/8Mq1e3o3j
clang 11 does reject it but clang 12 does NOT reject it.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/101557] the value of '<temporary>' is not usable in a constant expression
2021-07-21 15:13 [Bug c++/101557] New: the value of '<temporary>' is not usable in a constant expression federico.kircheis at gmail dot com
` (2 preceding siblings ...)
2021-07-21 16:07 ` pinskia at gcc dot gnu.org
@ 2021-07-21 16:15 ` federico.kircheis at gmail dot com
2023-04-16 9:31 ` federico at kircheis dot it
4 siblings, 0 replies; 6+ messages in thread
From: federico.kircheis at gmail dot com @ 2021-07-21 16:15 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101557
--- Comment #4 from Federico Kircheis <federico.kircheis at gmail dot com> ---
Indeed.
I just checked the latest versions.
I wonder if there is a common cause that makes this recursive data structure
harder to evaluate at compile time.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/101557] the value of '<temporary>' is not usable in a constant expression
2021-07-21 15:13 [Bug c++/101557] New: the value of '<temporary>' is not usable in a constant expression federico.kircheis at gmail dot com
` (3 preceding siblings ...)
2021-07-21 16:15 ` federico.kircheis at gmail dot com
@ 2023-04-16 9:31 ` federico at kircheis dot it
4 siblings, 0 replies; 6+ messages in thread
From: federico at kircheis dot it @ 2023-04-16 9:31 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101557
--- Comment #5 from Federico Kircheis <federico at kircheis dot it> ---
Today I just found a possible workaround that involves macros and lambdas...
----
struct node {
const char* d;
const node& left;
};
#define LEAF(a) []()-> const node&{ constexpr static auto a = node{"a",Null};
return a; }()
#define NODE(a,b) []()-> const node&{constexpr static auto a = node{"a",b};
return a; }()
constexpr node Null = node{"",Null};
constexpr auto a = node{"a",NODE(a,LEAF(b))};
constexpr auto a2 = node{"a",node{"a", node{"a", Null}}};
constexpr int mysize(const node& n) noexcept{
return (&n == &Null) ? 0 : 1 + mysize(n.left);
}
constexpr auto size0 = mysize(Null);
static_assert(size0 == 0, "");
constexpr auto size3 = mysize(a);
static_assert(size3 == 3, "");
----
"mysize(a)" compiles, while "mysize(a2)" does not, even if the two
datastructures are equivalent
Unfortunately clang does not accept it
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2023-04-16 9:31 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-07-21 15:13 [Bug c++/101557] New: the value of '<temporary>' is not usable in a constant expression federico.kircheis at gmail dot com
2021-07-21 15:24 ` [Bug c++/101557] " jakub at gcc dot gnu.org
2021-07-21 16:03 ` federico.kircheis at gmail dot com
2021-07-21 16:07 ` pinskia at gcc dot gnu.org
2021-07-21 16:15 ` federico.kircheis at gmail dot com
2023-04-16 9:31 ` federico at kircheis dot it
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