cmake: Report if the <LANG>_CLANG_TIDY tool exits with non-zero

When using `<LANG>_CLANG_TIDY` our internal launcher for the tool must
capture its return code and stderr and report them on failure.
Otherwise incorrect command lines silently fail.

Closes: #16435

Help/release/dev/capture-clang-tidy-errors.rst

@@ -0,0 +1,6 @@
+capture-clang-tidy-errors
+-------------------------
+
+* If a command specified by the :prop_tgt:`<LANG>_CLANG_TIDY` target property
+  returns non-zero at build time this is now treated as an error instead of
+  silently ignored.

Source/cmcmd.cxx

@@ -358,14 +358,21 @@ int cmcmd::ExecuteCMakeCommand(std::vector<std::string>& args)
 
         // Run the tidy command line.  Capture its stdout and hide its stderr.
         std::string stdOut;
-        if (!cmSystemTools::RunSingleCommand(tidy_cmd, &stdOut, CM_NULLPTR,
-                                             &ret, CM_NULLPTR,
+        std::string stdErr;
+        if (!cmSystemTools::RunSingleCommand(tidy_cmd, &stdOut, &stdErr, &ret,
+                                             CM_NULLPTR,
                                              cmSystemTools::OUTPUT_NONE)) {
-          std::cerr << "Error running '" << tidy_cmd[0] << "'\n";
+          std::cerr << "Error running '" << tidy_cmd[0] << "': " << stdErr
+                    << "\n";
           return 1;
         }
         // Output the stdout from clang-tidy to stderr
         std::cerr << stdOut;
+        // If clang-tidy exited with an error do the same.
+        if (ret != 0) {
+          std::cerr << stdErr;
+          return ret;
+        }
       }
       if (!lwyu.empty()) {
         // Construct the ldd -r -u (link what you use lwyu) command line

Tests/RunCMake/ClangTidy/C-bad-Build-result.txt

@@ -0,0 +1 @@
+[^0]

Tests/RunCMake/ClangTidy/C-bad-Build-stdout.txt

@@ -0,0 +1,2 @@
+stdout from bad command line arg '-bad'
+stderr from bad command line arg '-bad'

Tests/RunCMake/ClangTidy/C-bad.cmake

@@ -0,0 +1,3 @@
+enable_language(C)
+set(CMAKE_C_CLANG_TIDY "${PSEUDO_TIDY}" -bad)
+add_executable(main main.c)

Tests/RunCMake/ClangTidy/RunCMakeTest.cmake

@@ -20,3 +20,4 @@ if (NOT RunCMake_GENERATOR STREQUAL "Watcom WMake")
   run_tidy(C-launch)
   run_tidy(CXX-launch)
 endif()
+run_tidy(C-bad)

Tests/RunCMake/pseudo_tidy.c

@@ -1,9 +1,15 @@
 #include <stdio.h>
+#include <string.h>
 
 int main(int argc, char* argv[])
 {
   int i;
   for (i = 1; i < argc; ++i) {
+    if (strcmp(argv[i], "-bad") == 0) {
+      fprintf(stdout, "stdout from bad command line arg '-bad'\n");
+      fprintf(stderr, "stderr from bad command line arg '-bad'\n");
+      return 1;
+    }
     if (argv[i][0] != '-') {
       fprintf(stdout, "%s:0:0: warning: message [checker]\n", argv[i]);
       break;