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

1 files changed, 10 insertions, 3 deletions

diff --git a/Source/cmcmd.cxx b/Source/cmcmd.cxx
index dca5ffc..f1ce75a 100644
--- a/Source/cmcmd.cxx
+++ b/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