file(DOWNLOAD): Don't fail if given just a filename to write to

Fixes: #17969

Source/cmFileCommand.cxx

@@ -1707,7 +1707,8 @@ bool HandleDownloadCommand(std::vector<std::string> const& args,
   // as we receive downloaded bits from curl...
   //
   std::string dir = cmSystemTools::GetFilenamePath(file);
-  if (!cmSystemTools::FileExists(dir) && !cmSystemTools::MakeDirectory(dir)) {
+  if (!dir.empty() && !cmSystemTools::FileExists(dir) &&
+      !cmSystemTools::MakeDirectory(dir)) {
     std::string errstring = "DOWNLOAD error: cannot create directory '" + dir +
       "' - Specify file by full path name and verify that you "
       "have directory creation and file write privileges.";

Tests/CMakeTests/FileDownloadTest.cmake.in

@@ -145,3 +145,21 @@ file(DOWNLOAD
   )
 message(STATUS "${status}")
 __reportIfWrongStatus("${status}" 6) # 6 corresponds to an unresolvable host name
+
+message(STATUS "FileDownload:12")
+set(absFile "@CMAKE_CURRENT_BINARY_DIR@/file12.png")
+if(EXISTS "${absFile}")
+  file(RENAME ${absFile} "${absFile}_beingRemoved")
+  file(REMOVE "${absFile}_beingRemoved")
+endif()
+file(DOWNLOAD
+  ${url}
+  file12.png
+  TIMEOUT ${timeout}
+  EXPECTED_MD5 dbd330d52f4dbd60115d4191904ded92
+  STATUS status
+  )
+__reportIfWrongStatus("${status}" 0)
+if(NOT EXISTS file12.png)
+  message(SEND_ERROR "file12.png not downloaded: ${status}")
+endif()

Tests/CMakeTests/FileTestScript.cmake

@@ -183,7 +183,7 @@ elseif(testname STREQUAL to_native_path) # pass
 elseif(testname STREQUAL download_wrong_number_of_args) # fail
   file(DOWNLOAD zzzz://bogus/ffff)
 
-elseif(testname STREQUAL download_file_with_no_path) # fail
+elseif(testname STREQUAL download_file_with_no_path) # pass
   file(DOWNLOAD zzzz://bogus/ffff ffff)
 
 elseif(testname STREQUAL download_missing_time) # fail