[clangd] Fix test failure when it's built with compiler flags unknown by clang

[xry111]

Summary

If LLVM is built with a compiler other than clang, the compile_commands.json
file may contain compiler flags unknown by clang. When a clangd test is copied
into the build directory and checked, clangd will pick the unknown flag from
the file and cause a test failure. Create an empty compile_commands.json in
the test directory nested in the build directory to override it.

[thesamesam]

(I even struggled to find docs for the fix we used for clang-tidy, although it obviously works.)

Thank you!

[kadircet]

This broke buildbots because compile_commands.json being empty has other consequences.
Fixing forward in 9ffef0f24de0fc05b946e662a7b233a32ad058e3. Please don't land such infrastructure changes without approvals from code owners in the future.

[thesamesam]

Sure & thanks.

[xry111]

Hmm, I'd tested the change before creating this but it seems those tests are "UNSUPPORTED" on my system :(.

clang-tools-extra/clangd/test/CMakeLists.txt

 
 configure_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.py.in
   ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg.py
   MAIN_CONFIG
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.cfg.py
   )
 
+# Copy an empty compile_commands.json to override the compile_commands.json
+# in the top level build directory.  Or if a clangd test involves creating a
+# temporary source file in the build directory and run clangd to check it,
+# it can pick up unrecognizable command options when LLVM is built with
+# another compiler or a different version of Clang.
+configure_file(
+  ${CMAKE_CURRENT_SOURCE_DIR}/compile_commands.json
+  ${CMAKE_CURRENT_BINARY_DIR}/compile_commands.json
+)
+
 add_lit_testsuite(check-clangd "Running the Clangd regression tests"
   # clangd doesn't put unittest configs in test/unit like every other project.
   # Because of that, this needs to pass two folders here, while every other
   # project only needs to pass CMAKE_CURRENT_BINARY_DIR.
   ${CMAKE_CURRENT_BINARY_DIR}/../unittests;${CMAKE_CURRENT_BINARY_DIR}
   DEPENDS ${CLANGD_TEST_DEPS})

clang-tools-extra/clangd/test/compile_commands.json

+[]