Theory is that the include directories for mpi are not getting properly added due to the compiler wrapper. CMake code here is Claude output and is not suitable for merging as is, but fine for testing the theory.

Author: jacobmerson

CMakeLists.txt

@@ -34,14 +34,6 @@ if(PROJECT_IS_TOP_LEVEL)
   message(STATUS "RUN_CLANG_TIDY_EXECUTABLE= ${RUN_CLANG_TIDY_EXECUTABLE}")
   message(STATUS "CLANG_TIDY_CONFIG_FILE=${CLANG_TIDY_CONFIG_FILE}")
   message(STATUS "CLANG_TIDY_SOURCE_FILTER=${CLANG_TIDY_SOURCE_FILTER}")
-
-  add_custom_target(run-clang-tidy
-          VERBATIM COMMAND ${RUN_CLANG_TIDY_EXECUTABLE}
-          -clang-tidy-binary ${CLANG_TIDY_EXECUTABLE}
-          -p ${CMAKE_BINARY_DIR}
-          -config-file ${CLANG_TIDY_CONFIG_FILE}
-          -source-filter ${CLANG_TIDY_SOURCE_FILTER}
-  )
 endif()
 
 option(PCMS_ENABLE_SERVER "enable the coupling server implementation" ON)
@@ -82,6 +74,38 @@ endif()
 
 set(MPI_CXX_SKIP_MPICXX ON)
 find_package(MPI REQUIRED)
+
+# Create the run-clang-tidy target here (after find_package(MPI)) so we can
+# forward any MPI include paths that are missing from compile_commands.json.
+# When CMAKE_CXX_COMPILER is an MPI wrapper (e.g. mpicxx), FindMPI may leave
+# MPI_CXX_INCLUDE_DIRS empty because the wrapper handles includes implicitly,
+# meaning compile_commands.json has no explicit -I flags for mpi.h.
+if(PROJECT_IS_TOP_LEVEL)
+  set(_clang_tidy_extra_args "")
+  get_target_property(_mpi_cxx_incs MPI::MPI_CXX INTERFACE_INCLUDE_DIRECTORIES)
+  if(NOT _mpi_cxx_incs)
+    execute_process(
+      COMMAND ${MPI_CXX_COMPILER} -show
+      OUTPUT_VARIABLE _mpi_show
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      ERROR_QUIET)
+    string(REGEX MATCHALL "-I([^ ]+)" _mpi_inc_flags "${_mpi_show}")
+    foreach(_flag IN LISTS _mpi_inc_flags)
+      list(APPEND _clang_tidy_extra_args "-extra-arg=${_flag}")
+    endforeach()
+    message(STATUS "clang-tidy MPI extra args: ${_clang_tidy_extra_args}")
+  endif()
+
+  add_custom_target(run-clang-tidy
+    VERBATIM COMMAND ${RUN_CLANG_TIDY_EXECUTABLE}
+    -clang-tidy-binary ${CLANG_TIDY_EXECUTABLE}
+    -p ${CMAKE_BINARY_DIR}
+    -config-file ${CLANG_TIDY_CONFIG_FILE}
+    -source-filter ${CLANG_TIDY_SOURCE_FILTER}
+    ${_clang_tidy_extra_args}
+  )
+endif()
+
 message(STATUS "Found redev: ${redev_DIR} (found version ${redev_VERSION})")
 if(PCMS_ENABLE_OMEGA_H)
   find_package(Omega_h REQUIRED VERSION 10)