ForceToRelativePath: Fix spurious assertion when local path is root dir

Fixes: #19909

Source/cmSystemTools.cxx

@@ -1120,8 +1120,13 @@ std::string cmSystemTools::ForceToRelativePath(std::string const& local_path,
   assert(local_path.front() != '\"');
   assert(remote_path.front() != '\"');
 
-  // The local path should never have a trailing slash.
-  assert(local_path.empty() || local_path.back() != '/');
+  // The local path should never have a trailing slash except if it is just the
+  // bare root directory
+  assert(local_path.empty() || local_path.back() != '/' ||
+         local_path.size() == 1 ||
+         (local_path.size() == 3 && local_path[1] == ':' &&
+          ((local_path[0] >= 'A' && local_path[0] <= 'Z') ||
+           (local_path[0] >= 'a' && local_path[0] <= 'z'))));
 
   // If the path is already relative then just return the path.
   if (!cmSystemTools::FileIsFullPath(remote_path)) {

Tests/RunCMake/message/RunCMakeTest.cmake

@@ -5,6 +5,15 @@ run_cmake(nomessage)
 run_cmake(message-internal-warning)
 run_cmake(nomessage-internal-warning)
 run_cmake(warnmessage)
+
+# Have to explicitly give the command for the working dir to be honoured
+set(RunCMake_TEST_COMMAND_WORKING_DIRECTORY /)
+run_cmake_command(
+    warnmessage-rootdir
+    ${CMAKE_COMMAND} -P ${RunCMake_SOURCE_DIR}/warnmessage-rootdir.cmake
+  )
+unset(RunCMake_TEST_COMMAND_WORKING_DIRECTORY)
+
 # message command sets fatal occurred flag, so check each type of error
 
 # separately

Tests/RunCMake/message/warnmessage-rootdir-stderr.txt

@@ -0,0 +1 @@
+We expect to see this warning message

Tests/RunCMake/message/warnmessage-rootdir.cmake

@@ -0,0 +1,5 @@
+cmake_minimum_required(VERSION 3.15)
+
+# Generating the backtrace for this warning message used to trigger a
+# spurious assertion when the current directory is the root directory
+message(WARNING "We expect to see this warning message")