[analyzer] FP for cplusplus.PlacementNew

[puetzk]

#include <new>

int main() {
    void *buf = ::operator new(sizeof(int)*2);
    int * placement_int = new (buf) int[2];
    placement_int[0] = 42;
    ::operator delete(buf);
}

clang-tidy produces

<source>:5:27: warning: Storage provided to placement new is only 8 bytes, whereas the allocated array type requires more space for internal needs [clang-analyzer-cplusplus.PlacementNew]
    5 |     int * placement_int = new (buf) int[2];
      |                           ^~~~~~~~~~~~~~~~
<source>:4:5: note: 'buf' initialized here
    4 |     void *buf = ::operator new(sizeof(int)*2);
      |     ^~~~~~~~~
<source>:5:27: note: Storage provided to placement new is only 8 bytes, whereas the allocated array type requires more space for internal needs
    5 |     int * placement_int = new (buf) int[2];
      |                           ^~~~~~~~~~~~~~~~

Reproduced in https://godbolt.org/z/MYGc5qfn6 using clang version 22.0.0git (https://github.com/llvm/llvm-project.git 82d7405)

I think this warning is incorrect, as described at https://stackoverflow.com/questions/8720425/array-placement-new-requires-unspecified-overhead-in-the-buffer and in https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#2382 or https://cplusplus.github.io/CWG/issues/2382.html. This is a non-allocating placement new, in which case (per that defect resolution) no additional space may be requested.

So I think these "array overhead" warnings are always false-positives. However, they do seem to have been added in May 2020:
https://github.com/llvm/llvm-project/blame/1c541aa9f9a2453324724bfb9d661bc672778d10/clang/lib/StaticAnalyzer/Checkers/CheckPlacementNew.cpp#L120-L134 shows it came in with 7c37684 and https://reviews.llvm.org/D76996
That dates this first being added to clang after that DR had been reported, voted on, and the resolution accepted into CD5. So I don't think they are simply outdated; maybe the author/reviewers were unaware of the defect report?

Or maybe I'm misunderstanding something and this really is buggy code - "I'm wrong" is certainly possible too :-)

[llvmbot]

@llvm/issue-subscribers-clang-static-analyzer

Author: Kevin Puetz (puetzk)

```c++ #include <new>

int main() {
void *buf = ::operator new(sizeof(int)*2);
int * placement_int = new (buf) int[2];
placement_int[0] = 42;
::operator delete(buf);
}

clang-tidy produces

<source>:5:27: warning: Storage provided to placement new is only 8 bytes, whereas the allocated array type requires more space for internal needs [clang-analyzer-cplusplus.PlacementNew]
5 | int * placement_int = new (buf) int[2];
| ^~~~~~~~~~~~~~~~
<source>:4:5: note: 'buf' initialized here
4 | void *buf = ::operator new(sizeof(int)*2);
| ^~~~~~~~~
<source>:5:27: note: Storage provided to placement new is only 8 bytes, whereas the allocated array type requires more space for internal needs
5 | int * placement_int = new (buf) int[2];
| ^~~~~~~~~~~~~~~~

Reproduced in https://godbolt.org/z/MYGc5qfn6 using clang version 22.0.0git (https://github.com/llvm/llvm-project.git 82d7405b3bb911c86d0b07c8a6bec5044acbdf66)

I think this warning is incorrect, as described at https://stackoverflow.com/questions/8720425/array-placement-new-requires-unspecified-overhead-in-the-buffer and in https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#<!-- -->2382 or https://cplusplus.github.io/CWG/issues/2382.html. This is a non-allocating placement new, in which case (per that defect resolution) no additional space may be requested.

So I think these "array overhead" warnings are *always* false-positives. However, they do seem to have been added in May 2020:
https://github.com/llvm/llvm-project/blame/1c541aa9f9a2453324724bfb9d661bc672778d10/clang/lib/StaticAnalyzer/Checkers/CheckPlacementNew.cpp#L120-L134 shows it came in with https://github.com/llvm/llvm-project/commit/7c3768495e8c1599dc30986f7bd47d5e91f303f2 and https://reviews.llvm.org/D76996
That dates this first being added to clang **after** that DR had been reported, voted on, and the resolution accepted into CD5. So I don't think they are simply outdated; maybe the author/reviewers were unaware of the defect report?

Or maybe I'm misunderstanding something and this really is buggy code - "I'm wrong" is certainly possible too :-)


</details>

[steakhal]

Or maybe I'm misunderstanding something and this really is buggy code - "I'm wrong" is certainly possible too :-)

No, you interpret this right, and I share your opinion. Placement new never needs extra storage.

[steakhal]

This should be fairly easy to fix.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor is working on this issue. If someone is assigned to the issue or claimed to be working on it, ping the person. After one week without a response, the assignee may be changed.
2. Leave a comment indicating that you are working on the issue, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-analysis to only run Clang's Static Analyzer tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Kevin Puetz (puetzk)

```c++ #include <new>

int main() {
void *buf = ::operator new(sizeof(int)*2);
int * placement_int = new (buf) int[2];
placement_int[0] = 42;
::operator delete(buf);
}

clang-tidy produces

<source>:5:27: warning: Storage provided to placement new is only 8 bytes, whereas the allocated array type requires more space for internal needs [clang-analyzer-cplusplus.PlacementNew]
5 | int * placement_int = new (buf) int[2];
| ^~~~~~~~~~~~~~~~
<source>:4:5: note: 'buf' initialized here
4 | void *buf = ::operator new(sizeof(int)*2);
| ^~~~~~~~~
<source>:5:27: note: Storage provided to placement new is only 8 bytes, whereas the allocated array type requires more space for internal needs
5 | int * placement_int = new (buf) int[2];
| ^~~~~~~~~~~~~~~~

Reproduced in https://godbolt.org/z/MYGc5qfn6 using clang version 22.0.0git (https://github.com/llvm/llvm-project.git 82d7405b3bb911c86d0b07c8a6bec5044acbdf66)

I think this warning is incorrect, as described at https://stackoverflow.com/questions/8720425/array-placement-new-requires-unspecified-overhead-in-the-buffer and in https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#<!-- -->2382 or https://cplusplus.github.io/CWG/issues/2382.html. This is a non-allocating placement new, in which case (per that defect resolution) no additional space may be requested.

So I think these "array overhead" warnings are *always* false-positives. However, they do seem to have been added in May 2020:
https://github.com/llvm/llvm-project/blame/1c541aa9f9a2453324724bfb9d661bc672778d10/clang/lib/StaticAnalyzer/Checkers/CheckPlacementNew.cpp#L120-L134 shows it came in with https://github.com/llvm/llvm-project/commit/7c3768495e8c1599dc30986f7bd47d5e91f303f2 and https://reviews.llvm.org/D76996
That dates this first being added to clang **after** that DR had been reported, voted on, and the resolution accepted into CD5. So I don't think they are simply outdated; maybe the author/reviewers were unaware of the defect report?

Or maybe I'm misunderstanding something and this really is buggy code - "I'm wrong" is certainly possible too :-)


</details>

[abatef]

can you assign me to this

[aadanen]

Hi :)
I'm a new contributor and I would like to pick up this issue

Thanks
Aaron