👷 Add clang-tidy canary

Problem:
- `clang-tidy` exits with success if the `.clang-tidy` file is ill-formed.

Solution:
- Add a clang-tidy-canary target which tests if `clang-tidy` has any error
  output when run with `--verify-config`.

Author: elbeno

cmake/clang-tidy.cmake

@@ -100,31 +100,15 @@ function(clang_tidy_header_sets TARGET EXCLUDE_DIRS EXCLUDE_FILES
     endforeach()
 endfunction()
 
-function(clang_tidy_interface)
-    if(${ARGC} EQUAL 1)
-        set(CT_TARGET ${ARGV0})
-    else()
-        set(options "")
-        set(oneValueArgs TARGET)
-        set(multiValueArgs EXCLUDE_DIRS EXCLUDE_FILES EXCLUDE_FILESETS)
-        cmake_parse_arguments(CT "${options}" "${oneValueArgs}"
-                              "${multiValueArgs}" ${ARGN})
-    endif()
-
-    get_target_property(HEADER_SETS ${CT_TARGET} INTERFACE_HEADER_SETS)
-    if(HEADER_SETS)
-        message(
-            STATUS "Using INTERFACE_HEADER_SETS to clang-tidy ${CT_TARGET}.")
-        clang_tidy_header_sets(${CT_TARGET} "${CT_EXCLUDE_DIRS}"
-                               "${CT_EXCLUDE_FILES}" "${CT_EXCLUDE_FILESETS}")
-    else()
-        message(
-            NOTICE
-            "${CT_TARGET} has no INTERFACE_HEADER_SETS, using INTERFACE_INCLUDE_DIRECTORIES instead. "
-            "Use `target_sources` instead of `target_include_directories` to specify header sets for better control."
-        )
-        clang_tidy_dirs(${CT_TARGET} "${CT_EXCLUDE_DIRS}" "${CT_EXCLUDE_FILES}")
-    endif()
-
-    compute_branch_diff(clang-tidy ".hpp")
-endfunction()
+if(NOT TARGET clang-tidy-canary)
+    message(STATUS "Adding clang-tidy-canary target for ${CMAKE_SOURCE_DIR}")
+    add_custom_command(
+        OUTPUT clang_tidy_canary.alive
+        COMMAND ${CLANG_TIDY_PROGRAM} "--verify-config" 2>clang_tidy.log
+        COMMAND "!" "[" "-s" "clang_tidy.log" "]"
+        COMMAND ${CMAKE_COMMAND} "-E" "touch" "clang_tidy_canary.alive"
+        DEPENDS ${CMAKE_SOURCE_DIR}/.clang-tidy)
+    add_custom_target(clang-tidy-canary DEPENDS clang_tidy_canary.alive)
+    add_dependencies(clang-tidy clang-tidy-canary)
+    add_dependencies(clang-tidy-branch-diff clang-tidy-canary)
+endif()