| Issue |
157283
|
| Summary |
rejects valid on defaulted default constructor for 'std::vector<forwarded_class>'
|
| Labels |
c++,
rejects-valid
|
| Assignees |
|
| Reporter |
wheatman
|
I think the following valid code is being rejected
```cpp
#include <vector>
class forwarded_class;
struct foo {
std::vector<forwarded_class> v = {};
// std::vector<forwarded_class> v; // this one works
};
class forwarded_class {};
```
The error is
```
In file included from <source>:1:
In file included from /opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/vector:68:
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/stl_vector.h:376:35: error: arithmetic on a pointer to an incomplete type 'forwarded_class'
376 | _M_impl._M_end_of_storage - _M_impl._M_start);
| ~~~~~~~~~~~~~~~~~~~~~~~~~ ^
/opt/compiler-explorer/gcc-15.2.0/lib/gcc/x86_64-linux-gnu/15.2.0/../../../../include/c++/15.2.0/bits/stl_vector.h:561:7: note: in instantiation of member function 'std::_Vector_base<forwarded_class, std::allocator<forwarded_class>>::~_Vector_base' requested here
561 | vector() = default;
| ^
<source>:6:36: note: in defaulted default constructor for 'std::vector<forwarded_class>' first required here
6 | std::vector<forwarded_class> v = {};
| ^
<source>:3:7: note: forward declaration of 'forwarded_class'
3 | class forwarded_class;
| ^
1 error generated.
Compiler returned: 1
```
https://godbolt.org/z/qMv9q5zxd
This code compiles with gcc 15.2 and msvc19.43
It does not compile with clang 21.1 or current trunk (4d0b81616975c4ea8984170122a04553cb14337a)
I thought those two declarations should be the same, since the vector should be default initialized anyway.
One difference I notice is how they interplay with `-Wmissing-designated-field-initializers`
Where having the extra `{}` seems to not require me to specify that field to not receive the warning
```cpp
#include <vector>
class forwarded_class {}; // changed to make it compile and show the warning
struct foo1 {
int x;
std::vector<forwarded_class> v = {};
};
struct foo2 {
int x;
std::vector<forwarded_class> v;
};
void x() {
foo1 f{.x=0};
foo2 g{.x=0};
}
```
```
<source>:17:16: warning: missing field 'v' initializer [-Wmissing-designated-field-initializers]
17 | foo2 g{.x=0};
| ^
```
https://godbolt.org/z/7zYeaa68M
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