Possible false positive in undefined-inline with `__alloctr_rebind_helper`

[Axel-Naumann]

Bugzilla Link	20708
Version	trunk
OS	Linux
Attachments	excerpt from libstdc++ 4.9.1/bits/alloc_traits.h; reproducer
CC	@DougGregor

Extended Description

Hi,

with the attached file.h (excerpt of bits/alloc_traits.h from libstdc++ of GCC 4.9.1) and clang r212179 I get

$ clang -Wall -fsyntax-only -x c++ -std=c++11 file.h
file.h:28:3: warning: inline function '__alloctr_rebind_helper<int, float>::_S_chk<int, float>' is not defined [-Wundefined-inline]
                _S_chk(...);
                ^
file.h:31:32: note: used here
              using __type = decltype(_S_chk<_Alloc, _Tp>(nullptr));
                                      ^
1 warning generated.

I don't think decltype should require a definition; I don't see how a definition could change the decltype. It compiles just fine without the warning.

As I said this happens in the wild with libstdc++ of GCC 4.9.1 (installed side by side to the system compiler version) - where we still need to figure out why clang thinks they are not system headers. But that shouldn't affect this issue.

Thanks!

Axel.

[AaronBallman]

This issue was fixed in Clang 3.9: https://godbolt.org/z/Geb354dcG

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Axel Naumann (Axel-Naumann)

Details

| | | | --- | --- | | Bugzilla Link | [20708](https://llvm.org/bz20708) | | Version | trunk | | OS | Linux | | Attachments | [excerpt from libstdc++ 4.9.1/bits/alloc_traits.h; reproducer](https://user-images.githubusercontent.com/92601850/143749907-64b27860-fb08-41ff-9946-2a5961186606.gz) | | CC | @DougGregor |

Extended Description

Hi,

with the attached file.h (excerpt of bits/alloc_traits.h from libstdc++ of GCC 4.9.1) and clang r212179 I get

$ clang -Wall -fsyntax-only -x c++ -std=c++11 file.h
file.h:28:3: warning: inline function '__alloctr_rebind_helper<int, float>::_S_chk<int, float>' is not defined [-Wundefined-inline]
                _S_chk(...);
                ^
file.h:31:32: note: used here
              using __type = decltype(_S_chk<_Alloc, _Tp>(nullptr));
                                      ^
1 warning generated.

I don't think decltype should require a definition; I don't see how a definition could change the decltype. It compiles just fine without the warning.

As I said this happens in the wild with libstdc++ of GCC 4.9.1 (installed side by side to the system compiler version) - where we still need to figure out why clang thinks they are not system headers. But that shouldn't affect this issue.

Thanks!

Axel.