[clang-tidy] "unknown pragma ignored" generated on "pragma pack" in c++20 mode

[vient]

Source test.cpp

#pragma pack(1)
struct S {};

Compiling with clang++-17 -Wall -Werror -std=c++20 -o test.o -c test.cpp works fine, no errors.

I created /tmp/test/compile_commands.json using -MJ clang option output:

[
  {
    "directory": "/tmp/test",
    "file": "test.cpp",
    "output": "test.o",
    "arguments": [
      "/usr/lib/llvm-17/bin/clang",
      "-xc++",
      "test.cpp",
      "-o",
      "test.o",
      "--driver-mode=g++",
      "-Wall",
      "-Werror",
      "-std=c++20",
      "-c",
      "--target=x86_64-pc-linux-gnu"
    ]
  }
]

Running clang-tidy-17 test.cpp in /tmp/test folder produces following output

$ clang-tidy-17 test.cpp
1 error generated.
Error while processing /tmp/test/test.cpp.
test.cpp:1:9: error: unknown pragma ignored [clang-diagnostic-unknown-pragmas]
    1 | #pragma pack(1)
      |         ^
Found compiler error(s).

Error is only produced in c++20 and newer modes, like c++2b, but not in c++17 or older.

I am using latest 17 build from LLVM apt repo

$ clang++-17 --version
Ubuntu clang version 17.0.2 (++20230925113614+481358974fb0-1~exp1~20230925113629.44)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

$ clang-tidy-17 --version
Ubuntu LLVM version 17.0.2
  Optimized build.

[vient]

Can't get this error on CE https://godbolt.org/z/4vPYY6aY4, does it mean that this is not present in trunk?

[PiotrZSL]

Got it with:

clang-tidy-17 test.cpp --extra-arg=-std=c++20
1 warning generated.
/fpwork/llvm-project/test.cpp:1:9: warning: unknown pragma ignored [clang-diagnostic-unknown-pragmas]
    1 | #pragma pack(1)
      |         ^

or

clang-tidy-17 test.cpp -- -std=c++20 -Wall
1 warning generated.
/fpwork/llvm-project/test.cpp:1:9: warning: unknown pragma ignored [clang-diagnostic-unknown-pragmas]
    1 | #pragma pack(1)
      |         ^

With trunk version is reproduced only when '--enable-module-headers-parsing' is used.
This is known issue caused by two preprocessors created for C++20 and above.
This functionality is disabled by default in clang-tidy 18.

[vient]

Ok, thanks for info, added this to my .clang-tidy

ExtraArgs:
- -Wno-unknown-pragmas  # TODO: remove this after switch to clang-tidy-18

[spt29]

Is this the fix? 992fa7b Any chance of a backport into the 17.x release branch?

[PiotrZSL]

More this would be a workaround: b530eee
And I think there is zero chance to get this backported.