https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110981
Bug ID: 110981
Summary: constexpr variable definition that requires dynamic
destruction should be rejected
Product: gcc
Version: 13.2.1
Status: UNCONFIRMED
Keywords: accepts-invalid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: de34 at live dot cn
Target Milestone: ---
The following program is accepted by GCC and results in dynamic destruction
codes (because the lifetime-extended unique_ptr temporary object is not
trivially destructible and has no constant destruction)
(https://godbolt.org/z/8e9Y9E9jf):
```
#include <memory>
constexpr std::unique_ptr<int>&& r = {};
int main()
{
r = std::make_unique<int>(42);
}
```
I think we should reject the definition of r due to CWG2529
(https://cplusplus.github.io/CWG/issues/2529.html).
Also, a slightly different example is also currently accepted by GCC
(https://godbolt.org/z/1rf768aKs):
```
#include <memory>
struct S {
std::unique_ptr<int>&& r;
};
constexpr S s{{}};
int main()
{
s.r = std::make_unique<int>(42);
}
```
This seemly also needs to be rejected, although there's an open CWG issue
(https://cplusplus.github.io/CWG/issues/2702.html).
The rationale should be that
- every lifetime-extended temporary object created in the initialization of a
constexpr variable needs to have constant destruction, and
- such a non-const temporary object is not usable in constant expressions, so
lvalue-to-rvalue conversion from any of its subobjects makes the destruction
non-constant.
