Now showing red squiggle under diagnostic location

autoload/youcompleteme.vim

@@ -75,6 +75,7 @@ function! youcompleteme#Enable()
   endif
 
   call s:SetUpSigns()
+  call s:SetUpSyntaxHighlighting()
 
   if g:ycm_allow_changing_updatetime
     set ut=2000
@@ -194,6 +195,28 @@ function! s:SetUpSigns()
 endfunction
 
 
+function! s:SetUpSyntaxHighlighting()
+  " We try to ensure backwards compatibility with Syntastic if the user has
+  " already defined styling for Syntastic highlight groups.
+
+  if !hlexists( 'YcmErrorSection' )
+    if hlexists( 'SyntasticError' )
+      highlight link YcmErrorSection SyntasticError
+    else
+      highlight link YcmErrorSection SpellBad
+    endif
+  endif
+
+  if !hlexists( 'YcmWarningSection' )
+    if hlexists( 'SyntasticWarning' )
+      highlight link YcmWarningSection SyntasticWarning
+    else
+      highlight link YcmWarningSection SpellCap
+    endif
+  endif
+endfunction
+
+
 function! s:SetUpBackwardsCompatibility()
   let complete_in_comments_and_strings =
         \ get( g:, 'ycm_complete_in_comments_and_strings', 0 )

python/ycm/diagnostic_interface.py

@@ -44,6 +44,7 @@ class DiagnosticInterface( object ):
     self._buffer_number_to_line_to_diags = _ConvertDiagListToDict( diags )
     self._next_sign_id = _UpdateSigns( self._buffer_number_to_line_to_diags,
                                        self._next_sign_id )
+    _UpdateSquiggles( self._buffer_number_to_line_to_diags )
 
 
   def _EchoDiagnosticForLine( self, line_num ):
@@ -56,6 +57,17 @@ class DiagnosticInterface( object ):
     vimsupport.EchoTextVimWidth( diags[ 0 ][ 'text' ] )
 
 
+def _UpdateSquiggles( buffer_number_to_line_to_diags ):
+  vimsupport.ClearYcmSyntaxMatches()
+  line_to_diags = buffer_number_to_line_to_diags[ vim.current.buffer.number ]
+
+  for diags in line_to_diags.itervalues():
+    for diag in diags:
+      vimsupport.AddDiagnosticSyntaxMatch( diag[ 'lnum' ],
+                                           diag[ 'col' ],
+                                           _DiagnosticIsError( diag ) )
+
+
 def _UpdateSigns( buffer_number_to_line_to_diags, next_sign_id ):
   for buffer_number, line_to_diags in buffer_number_to_line_to_diags.iteritems():
     if not vimsupport.BufferIsVisible( buffer_number ):
@@ -67,7 +79,7 @@ def _UpdateSigns( buffer_number_to_line_to_diags, next_sign_id ):
         vimsupport.PlaceSign( next_sign_id,
                               line,
                               buffer_number,
-                              diag[ 'type' ] == 'E' )
+                              _DiagnosticIsError( diag ) )
         next_sign_id += 1
   return next_sign_id
 
@@ -83,3 +95,7 @@ def _ConvertDiagListToDict( diag_list ):
       diags.sort( key = lambda diag: itemgetter( 'col', 'type' ) )
   return buffer_to_line_to_diags
 
+
+def _DiagnosticIsError( diag ):
+  return diag[ 'type' ] == 'E'
+

python/ycm/vimsupport.py

@@ -107,6 +107,7 @@ def GetBufferFilepath( buffer_object ):
   return os.path.join( os.getcwd(), str( buffer_object.number ) )
 
 
+# TODO: only unplace our signs, not all signs
 def UnplaceAllSignsInBuffer( buffer_number ):
   if buffer_number < 0:
     return
@@ -119,6 +120,20 @@ def PlaceSign( sign_id, line_num, buffer_num, is_error = True ):
     sign_id, line_num, sign_name, buffer_num ) )
 
 
+def ClearYcmSyntaxMatches():
+  matches = VimExpressionToPythonType( 'getmatches()' )
+  for match in matches:
+    if match[ 'group' ].startswith( 'Ycm' ):
+      vim.eval( 'matchdelete({0})'.format( match[ 'id' ] ) )
+
+
+# Returns the ID of the newly added match
+def AddDiagnosticSyntaxMatch( line_num, column_num, is_error ):
+  group = 'YcmErrorSection' if is_error else 'YcmWarningSection'
+  return GetIntValue(
+    "matchadd('{0}', '\%{1}l\%{2}c')".format( group, line_num, column_num ) )
+
+
 # Given a dict like {'a': 1}, loads it into Vim as if you ran 'let g:a = 1'
 # When |overwrite| is True, overwrites the existing value in Vim.
 def LoadDictIntoVimGlobals( new_globals, overwrite = True ):