<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/133453>133453</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang-tidy] `HeaderFilterRegex` does not work if `.clang-tidy` is in subdirectory
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang-tidy
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          tearfur
      </td>
    </tr>
</table>

<pre>
    Specifying `HeaderFilterRegex: .*` in the `.clang-tidy` file in a subdirectory doesn't seem to take effect during compilation, but running `clang-tidy --dump-config` in the subdirectory does show `HeaderFilterRegex: '.*'`.

It works as expected if I move the `.clang-tidy` file to the project root, then clean and build.

The output of running `clang-tidy --dump-config` in the subdirectory is the same no matter `.clang-tidy` is placed in the project root or the subdirectory.

This CMake project demonstrates the issue:

[header_filter_subdir.zip](https://github.com/user-attachments/files/19507895/header_filter_subdir.zip)

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

Only relevant command output are shown.

#### In host machine

```console
$ docker run -it fedora:41 bash
$ docker cp header_filter_subdir/ ${CONTAINER}:/root
```

#### In Docker container

```console
# 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 316bb89c942c1a1cf61d3e673030f82d6f0b8acf)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
# cd ~/header_filter_subdir/
# 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/header_filter_subdir/build
# cd build/
# cmake --build .
[2/4] Building CXX object print/CMakeFiles/my_print.dir/print.cpp.o
/root/header_filter_subdir/print/print.cpp:5:21: warning: parameter 'i' is const-qualified in the function declaration; const-qualification of parameters only has an effect in function definitions [readability-avoid-const-params-in-decls]
    5 | void print_impl(int const i);
      |                     ^~~~~
[4/4] Linking CXX executable header_filter_subdir
# cd ../print/
# clang-tidy --dump-config | tee subdir.out
# mv .clang-tidy ..
# cd -
/root/header_filter_subdir/build
# cmake --build . --target clean
# cmake --build .
[1/4] Building CXX object CMakeFiles/header_filter_subdir.dir/main.cpp.o
/root/header_filter_subdir/print/print.h:4:16: warning: parameter 'i' is const-qualified in the function declaration; const-qualification of parameters only has an effect in function definitions [readability-avoid-const-params-in-decls]
    4 | void print(int const i);
 |                ^~~~~
[2/4] Building CXX object print/CMakeFiles/my_print.dir/print.cpp.o
/root/header_filter_subdir/print/print.cpp:5:21: warning: parameter 'i' is const-qualified in the function declaration; const-qualification of parameters only has an effect in function definitions [readability-avoid-const-params-in-decls]
    5 | void print_impl(int const i);
      | ^~~~~
/root/header_filter_subdir/print/print.h:4:16: warning: parameter 'i' is const-qualified in the function declaration; const-qualification of parameters only has an effect in function definitions [readability-avoid-const-params-in-decls]
    4 | void print(int const i);
 |                ^~~~~
[4/4] Linking CXX executable header_filter_subdir
# cd -
/root/header_filter_subdir/print
# clang-tidy --dump-config | tee rootdir.out
# diff subdir.out rootdir.out # no output
# grep -P ^HeaderFilterRegex subdir.out
HeaderFilterRegex: '.*'
```

</details>
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJzsWEtv67gV_jX05kCGRElWvPDCj3gmbe4D96bF7SqgxSObE4lUSSqPWeS3FyTlWJ7YSabTRTGYIIAl8vC8-PHj0WHGiK1EnJF8QfLViHV2p_TMItNVp0cbxZ9m31ssRfUk5BbIJP4ZGUe9FrVF_Q23-EjSOYwJnZNJDEKC3aETG5c1k9vICv7kJipRo5tlYLoNFxpLq_QTcIVGElpYMIgNWAWW3SFgVWFpgXfaGS1V04qaWaEkoUvYdBZ0J2Xvz8EORBHvmjYqlazEduDOK5NgdurhbDCEFj4eWrg4SDwn8fzKwoPSdwaYAXxssbTIQVRwBY26x7eCdjHtEFqtfnExaaWsi8LuUEJZI5PAJIdNJ2re27rZIajOtp0FVf2RUIUJY6xBkAoaZi3qE34KA23NSheRfOUsKP1K84ujwsDyk9ux_QqOjZLGamYxGBfGdEjSeVhB8sXOp_y28jm_DVrHv4qW5CtCL3bWtsaJ0zWh662wu24zLlVD6LozqCNmLSt3DUprCF27BLvfZJrHxcU0J3R9Vj2d9i6kS46WidqQ9DK8m65pmH4i6eXPqBGYRmieIEghB42tVrwrEYxF592S0PVhTVD7RdZPoLHGeyatg2zjtrXfRqfRQU72iSM0ffmHKwk7ZSw0rNwJib3EJA7_pZJG1X6UZsBVeYfaYQIiYaFCrjQj6TxLYMPM7liqbOFUNghdA6EZKRbLL59v5lefL7-RYhVy7tE5sH7a3VWvX0nLhET9hssplLx_4LKCruXM4mBASGNZXQPJ4jLieI-1at3uQtk4WG2FBSnkLyzyB6Rf6EbLWkmEN-BS1_f7n6hH53gr7ItXMJxx6_sJbzf67qchWoSTCdFP8Nn5AdHq-vqfn24vP88X15e3X799-dvl8uY7SVeEFv5UkXRR15yki_6MKVWbCB-tZoQWEK2Wn-Z_v7xd_OPqenV786-vlyRdfcMamTkkLNh85VPIAYx_M7pP4cu4swtRdI_aOMqM52GkfweajONx7HL49nk7k0BIk8lmczEtpxktE5aU1SThKU6KNE7j6oLySRVvLlhZhRN3w_QWraPVx4vJ7SSLOnkn1YOMaiG7x2grO08kGhmHRnGsnWirjHh0rBtCQ74SOjDzujMOwLUqWU3oeiPkIWvPZ47_IZHLJUlXIRvLHz_2z4QuCF3s83lix9_csigCR9jL_ppCDYKjtKISpb-yHLsuvcmQ-MGSHz9-36IVWiytuwwG1uaLKxCyUh8QgQi4kr3Tyx2Wd1Ap7S-24xVnc03XPbrA3Im2RX7WKlTIbKfRvC9x5NZAcJies1GeEnovzsGa9yLtsXE-3oOu8xGfkDly8Vop75hzcnAzvzEHEVSqk4EQi7cCCKpo4aqaAyfQgsRzR2R7DnihAHfcx0pv_ds0XNwAXvQVgfipL60VjfgVBzWMy7ovTEL95gJ1duJxagIpRBH8hBI1s8fz8WF-4Q-hv-Bhx-4RNogSHrSwFiVY1W9cKKfOHfvhpfEhWs0XlNB1RvJVcGC_fWrjq5tWC-nM-Zpn3RcfzdOtHx8Hm-G5bNux8nbecXGv8mUZSec5Sec0cRE-MO2KP8-JTLMGfQFHC-F2VBh3BRsb_btjtajEoX6rOll6GuFY1kyH0jldHIv3TKOqg2oDypUxO2aAyX0VLuRQXyWkcI8GSL5wrM02ohb2KWL3SvAoWPAKTSRk5BwwrrRzUAHIgRRLcJIhl7eiaWtCL4SvmaSxIDzqFr08ePlTfyS_fH5-fg6blu037VrIlyOOj1h2lm1qPF0GvcBiPB7sw_ASPVFqe38s7svhser2FUVzD4Oy2il9MRB9BAhHWD1GJkSR9ddo-GB4C77JW_A9gu3JQjm40jAh_1sE71w1StJ5Mvkz4jf7DX7PQfcEao8A-xfL_P-xzGCD_oL8_x7yf4yjP0Shwb8PErhTdczgXFTVgNeHEuDmpeq_5_efoRpbiL66MF_1kY7vh_faTK-_uH2P4dCnGPFZyqfplI1wlhRZmhTZlKaj3aws8zKrEj69qKZZNimzrCwqTnM2zVPOeToSMxrTPE7pRZInOU3HSZFk1TTjVcHigiYTksXYMFGP95XfyLdsZkmaZnk6qtkGa-Pbg5QOK0pK8tVIz_zn4abbGpLFtTDWHPRYYWvfWBwsy1en226TODTmpAqNNhDV6UaVkEedqFGn69nv-4gldO0j9I2jEOT9jP4nAAD__92N1LA">