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* [Bug c++/112590] New: structural constexpr class fails to instantiate
@ 2023-11-17 15:26 janezz55 at gmail dot com
2023-11-17 17:46 ` [Bug c++/112590] " mital at mitalashok dot co.uk
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: janezz55 at gmail dot com @ 2023-11-17 15:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112590
Bug ID: 112590
Summary: structural constexpr class fails to instantiate
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: janezz55 at gmail dot com
Target Milestone: ---
Compiling:
#include <cstddef>
template <auto C> void test() {}
template <std::size_t N>
struct S
{
int a_[N];
int* f_;
constexpr S(): a_{}, f_{a_} {}
};
int main()
{
test<S<10>{}>();
return 0;
}
fails with:
prog.cc: In function 'int main()':
prog.cc:18:16: error: no matching function for call to 'test<S<10>()>()'
18 | test<S<10>{}>();
| ~~~~~~~~~~~~~^~
prog.cc:4:6: note: candidate: 'template<auto C> void test()'
4 | void test()
| ^~~~
prog.cc:4:6: note: template argument deduction/substitution failed:
prog.cc:18:16: error: 'S<10>{int [10](), ((int*)(&<anonymous>.S<10>::a_))}' is
not a constant expression
18 | test<S<10>{}>();
| ~~~~~~~~~~~~~^~
prog.cc:18:16: error: 'S<10>()' is not a constant expression because it refers
to an incompletely initialized variable
But everything is initialized. I believe a workaround exists, but I didn't
experiment.
https://wandbox.org/permlink/xA81TIzI5XsXdh57
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c++/112590] structural constexpr class fails to instantiate
2023-11-17 15:26 [Bug c++/112590] New: structural constexpr class fails to instantiate janezz55 at gmail dot com
@ 2023-11-17 17:46 ` mital at mitalashok dot co.uk
2023-11-17 18:42 ` janezz55 at gmail dot com
2023-11-17 19:26 ` janezz55 at gmail dot com
2 siblings, 0 replies; 4+ messages in thread
From: mital at mitalashok dot co.uk @ 2023-11-17 17:46 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112590
Mital Ashok <mital at mitalashok dot co.uk> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |mital at mitalashok dot co.uk
--- Comment #1 from Mital Ashok <mital at mitalashok dot co.uk> ---
See [temp.arg.nontype]p3
<https://timsong-cpp.github.io/cppwp/n4868/temp.arg.nontype#3>:
> For a non-type template-parameter of reference type, or for each non-static
> data member of reference or pointer type in a non-type template-parameter
> of class type or subobject thereof, the reference or pointer value shall
> not refer to or be the address of (respectively):
> - A temporary object,
> - [...]
> - a subject of one of the above.
And "f_{a_}" is initializing the pointer as a subobject of a temporary
object, so this is invalid. Though the error given might be wrong or
misleading.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c++/112590] structural constexpr class fails to instantiate
2023-11-17 15:26 [Bug c++/112590] New: structural constexpr class fails to instantiate janezz55 at gmail dot com
2023-11-17 17:46 ` [Bug c++/112590] " mital at mitalashok dot co.uk
@ 2023-11-17 18:42 ` janezz55 at gmail dot com
2023-11-17 19:26 ` janezz55 at gmail dot com
2 siblings, 0 replies; 4+ messages in thread
From: janezz55 at gmail dot com @ 2023-11-17 18:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112590
--- Comment #2 from Janez Zemva <janezz55 at gmail dot com> ---
Very nice, but if I write:
int main()
{
static constexpr S<10> s;
return 0;
}
there will still be a compile error.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c++/112590] structural constexpr class fails to instantiate
2023-11-17 15:26 [Bug c++/112590] New: structural constexpr class fails to instantiate janezz55 at gmail dot com
2023-11-17 17:46 ` [Bug c++/112590] " mital at mitalashok dot co.uk
2023-11-17 18:42 ` janezz55 at gmail dot com
@ 2023-11-17 19:26 ` janezz55 at gmail dot com
2 siblings, 0 replies; 4+ messages in thread
From: janezz55 at gmail dot com @ 2023-11-17 19:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112590
--- Comment #3 from Janez Zemva <janezz55 at gmail dot com> ---
Sorry for my last comment. I have prepared a more involved example:
https://github.com/user1095108/ca/blob/master/consttests.cpp
If
test<a>();
is commented out, everything compiles, otherwise not.
^ permalink raw reply [flat|nested] 4+ messages in thread
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2023-11-17 15:26 [Bug c++/112590] New: structural constexpr class fails to instantiate janezz55 at gmail dot com
2023-11-17 17:46 ` [Bug c++/112590] " mital at mitalashok dot co.uk
2023-11-17 18:42 ` janezz55 at gmail dot com
2023-11-17 19:26 ` janezz55 at gmail dot com
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