[clang-tidy] `readability-avoid-const-params-in-decls` does not work in header files

[tearfur]

As the title suggests, readability-avoid-const-params-in-decls does not work in header files.

It can be demonstrated using this CMake project:

readability-avoid-const-params-in-decls.zip

It contains 2 functions:

• void print(int const): Declared in header, defined in source file.
• void print2(int const): Declared and defined in source file.

If you try to build it, you'll notice that readability-avoid-const-params-in-decls only warns about print2(), but I expect it to warn about both print() and print2().

P.S. I wish I can demonstrate this on Godbolt, but seems like running clang-tidy with a CMake project is not possible there.

Here are my detailed reproduce steps

Only relevant command output are shown.

In host machine

$ docker run -it fedora:41 bash
$ docker cp readability-avoid-const-params-in-decls/ <CONTAINER>:/root

In Docker container

# cd
# dnf update
# dnf install @c-development cmake git ninja-build
# git clone https://github.com/llvm/llvm-project.git
# cd llvm-project/
# cmake -S llvm -B build -G Ninja -DLLVM_ENABLE_PROJECTS='clang;lld;clang-tools-extra' -DCMAKE_BUILD_TYPE=Release
# cd build/
# cmake --build .
# cmake --install .
# clang --version
clang version 21.0.0git (https://github.com/llvm/llvm-project.git a629b505757a1853e6083290e5d8d7b82f4f4d4a)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
# cd ~/readability-avoid-const-params-in-decls/
# CC=clang CXX=clang++ cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Release
-- The C compiler identification is Clang 21.0.0
-- The CXX compiler identification is Clang 21.0.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/local/bin/clang - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/local/bin/clang++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Looking for clang-tidy
-- Looking for clang-tidy - found
'/usr/local/bin/clang-tidy' '--version'
LLVM (http://llvm.org/):
  LLVM version 21.0.0git
  Optimized build.
-- Configuring done (0.3s)
-- Generating done (0.0s)
-- Build files have been written to: /root/readability-avoid-const-params-in-decls/build
# cd build/
# cmake --build .
[2/3] Building CXX object CMakeFiles/readability_avoid_const_params_in_decls.dir/main.cpp.o
/root/readability-avoid-const-params-in-decls/main.cpp:5:17: warning: parameter 'i' is const-qualified in the function declaration; const-qualification of parameters only has an effect in function definitio
ns [readability-avoid-const-params-in-decls]
    5 | void print2(int const i);
      |                 ^~~~~
[3/3] Linking CXX executable readability_avoid_const_params_in_decls

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Yat Ho (tearfur)

Details

As the title suggests, `readability-avoid-const-params-in-decls` does not work in header files.

It can be demonstrated using this CMake project:

readability-avoid-const-params-in-decls.zip

It contains 2 functions:

• void print(int const): Declared in header, defined in source file.
• void print2(int const): Declared and defined in source file.

If you try to build it, you'll notice that readability-avoid-const-params-in-decls only warns about print2(), but I expect it to warn about both print() and print2().

P.S. I wish I can demonstrate this on Godbolt, but seems like running clang-tidy with a CMake project is not possible there.

<details>
<summary>Here are my detailed reproduce steps</summary>

Only relevant command output are shown.

In host machine

$ docker run -it fedora:41 bash
$ docker cp readability-avoid-const-params-in-decls/ <CONTAINER>:/root

In Docker container

# cd
# dnf update
# dnf install @<!-- -->c-development cmake git ninja-build
# git clone https://github.com/llvm/llvm-project.git
# cd llvm-project/
# cmake -S llvm -B build -G Ninja -DLLVM_ENABLE_PROJECTS='clang;lld;clang-tools-extra' -DCMAKE_BUILD_TYPE=Release
# cd build/
# cmake --build .
# cmake --install .
# clang --version
clang version 21.0.0git (https://github.com/llvm/llvm-project.git a629b505757a1853e6083290e5d8d7b82f4f4d4a)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
# cd ~/readability-avoid-const-params-in-decls/
# CC=clang CXX=clang++ cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Release
-- The C compiler identification is Clang 21.0.0
-- The CXX compiler identification is Clang 21.0.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/local/bin/clang - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/local/bin/clang++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Looking for clang-tidy
-- Looking for clang-tidy - found
'/usr/local/bin/clang-tidy' '--version'
LLVM (http://llvm.org/):
  LLVM version 21.0.0git
  Optimized build.
-- Configuring done (0.3s)
-- Generating done (0.0s)
-- Build files have been written to: /root/readability-avoid-const-params-in-decls/build
# cd build/
# cmake --build .
[2/3] Building CXX object CMakeFiles/readability_avoid_const_params_in_decls.dir/main.cpp.o
/root/readability-avoid-const-params-in-decls/main.cpp:5:17: warning: parameter 'i' is const-qualified in the function declaration; const-qualification of parameters only has an effect in function definitio
ns [readability-avoid-const-params-in-decls]
    5 | void print2(int const i);
      |                 ^~~~~
[3/3] Linking CXX executable readability_avoid_const_params_in_decls

</details>

[carlosgalvezp]

You need to add -header-filter=.* to the clang-tidy call to show issues from headers. Your example works just fine outside CMake:

$ cat foo.cpp 
#include "foo.h"

void f(int const);

$ cat foo.h
void f2(int const);

$ clang-tidy -checks=readability-avoid-const-params-in-decls foo.cpp -header-filter=.* --
2 warnings generated.
foo.cpp:3:12: warning: parameter 1 is const-qualified in the function declaration; const-qualification of parameters only has an effect in function definitions [readability-avoid-const-params-in-decls]
    3 | void f(int const);
      |            ^~~~~
foo.h:1:13: warning: parameter 1 is const-qualified in the function declaration; const-qualification of parameters only has an effect in function definitions [readability-avoid-const-params-in-decls]
    1 | void f2(int const);
      |             ^~~~~

[carlosgalvezp]

This change makes it work:

-   set(CMAKE_CXX_CLANG_TIDY "${CLANG_TIDY}"                   -checks=readability-avoid-const-params-in-decls)
+   set(CMAKE_CXX_CLANG_TIDY "${CLANG_TIDY}" -header-filter=.* -checks=readability-avoid-const-params-in-decls)

[tearfur]

Oh, that's what it was.

TIL, thanks!