Now checking ycm_core version for compatibility

autoload/youcompleteme.vim

@@ -37,6 +37,20 @@ function! youcompleteme#Enable()
     return
   endif
 
+  py import sys
+  py import vim
+  exe 'python sys.path.insert( 0, "' . s:script_folder_path . '/../python" )'
+  py import ycm
+
+  if !pyeval( 'ycm.CompatibleWithYcmCore()')
+    echohl WarningMsg |
+      \ echomsg "YouCompleteMe unavailable, ycm_core too old, PLEASE RECOMPILE" |
+      \ echohl None
+    return
+  endif
+
+  py ycm_state = ycm.YouCompleteMe()
+
   augroup youcompleteme
     autocmd!
     autocmd CursorMovedI * call s:OnCursorMovedInsertMode()
@@ -60,12 +74,6 @@ function! youcompleteme#Enable()
     set ut=2000
   endif
 
-  py import sys
-  py import vim
-  exe 'python sys.path.insert( 0, "' . s:script_folder_path . '/../python" )'
-  py import ycm
-  py ycm_state = ycm.YouCompleteMe()
-
   " Calling this once solves the problem of BufRead/BufEnter not triggering for
   " the first loaded file. This should be the last command executed in this
   " function!
@@ -477,7 +485,7 @@ function! s:ForceCompile()
   if !pyeval( 'ycm_state.NativeFiletypeCompletionUsable()' )
     echom "Native filetype completion not supported for current file, "
           \ . "cannot force recompilation."
-    return
+    return 0
   endif
 
   echom "Forcing compilation, this will block Vim until done."

cpp/ycm/ycm_core.cpp

@@ -41,6 +41,13 @@ bool HasClangSupport()
 #endif // USE_CLANG_COMPLETER
 }
 
+int YcmCoreVersion()
+{
+  // We increment this every time when we want to force users to recompile
+  // ycm_core.
+  return 1;
+}
+
 
 BOOST_PYTHON_MODULE(ycm_core)
 {
@@ -49,6 +56,7 @@ BOOST_PYTHON_MODULE(ycm_core)
 
   def( "HasClangSupport", HasClangSupport );
   def( "FilterAndSortCandidates", FilterAndSortCandidates );
+  def( "YcmCoreVersion", YcmCoreVersion );
 
   class_< IdentifierCompleter, boost::noncopyable >( "IdentifierCompleter" )
     .def( "EnableThreading", &IdentifierCompleter::EnableThreading )

python/ycm.py

@@ -33,6 +33,7 @@ except ImportError, e:
     'the docs. Full error: {1}'.format(
       os.path.dirname( os.path.abspath( __file__ ) ), str( e ) ) )
 
+
 from completers.all.identifier_completer import IdentifierCompleter
 from completers.all.omni_completer import OmniCompleter
 
@@ -237,3 +238,13 @@ def CurrentIdentifierFinished():
     return line[ : current_column ].isspace()
 
 
+COMPATIBLE_WITH_CORE_VERSION = 1
+
+def CompatibleWithYcmCore():
+  try:
+    current_core_version = ycm_core.YcmCoreVersion()
+  except AttributeError:
+    return False
+
+  return current_core_version == COMPATIBLE_WITH_CORE_VERSION
+