Add support for DocumentSymbol in document outline requests

We are intentionally not advertising the capability.
We do want a flat response, so receiving a DocumentSymbol is a
pessimisation.
Not advertising the capability means that conforming servers take the
faster code path and the likes of OmniSharp, that assume capabilities,
still work.

Yes, it's messy, but so is LSP.

ycmd/completers/language_server/language_server_completer.py

@@ -2655,10 +2655,10 @@ def GoToDocumentOutline( self, request_data ):
 
     result = response.get( 'result' ) or []
 
-    # We should only receive SymbolInformation (not DocumentSymbol)
     if any( 'range' in s for s in result ):
-      raise ValueError(
-        "Invalid server response; DocumentSymbol not supported" )
+      LOGGER.debug( 'Hierarchical DocumentSymbol not supported.' )
+      result = _FlattenDocumentSymbolHierarchy( result )
+      return _DocumentSymboListToGoTo( request_data, result )
 
     return _SymbolInfoListToGoTo( request_data, result )
 
@@ -3427,6 +3427,50 @@ def BuildGoToLocationFromSymbol( symbol ):
     return locations
 
 
+def _FlattenDocumentSymbolHierarchy( symbols ):
+  result = []
+  for s in symbols:
+    partial_results = [ s ]
+    if s.get( 'children' ):
+      partial_results.extend(
+        _FlattenDocumentSymbolHierarchy( s[ 'children' ] ) )
+    result.extend( partial_results )
+  return result
+
+
+def _DocumentSymboListToGoTo( request_data, symbols ):
+  """Convert a list of LSP DocumentSymbol into a YCM GoTo response"""
+
+  print(1)
+  def BuildGoToLocationFromSymbol( symbol ):
+    symbol[ 'uri' ] = lsp.FilePathToUri( request_data[ 'filepath' ] )
+    location, line_value = _LspLocationToLocationAndDescription(
+      request_data,
+      symbol )
+
+    description = ( f'{ lsp.SYMBOL_KIND[ symbol[ "kind" ] ] }: '
+                    f'{ symbol[ "name" ] }' )
+
+    goto = responses.BuildGoToResponseFromLocation( location,
+                                                    description )
+    goto[ 'extra_data' ] = {
+      'kind': lsp.SYMBOL_KIND[ symbol[ 'kind' ] ],
+      'name': symbol[ 'name' ],
+    }
+    return goto
+
+  locations = [ BuildGoToLocationFromSymbol( s ) for s in
+                sorted( symbols,
+                        key = lambda s: ( s[ 'kind' ], s[ 'name' ] ) ) ]
+
+  if not locations:
+    raise RuntimeError( "Symbol not found" )
+  elif len( locations ) == 1:
+    return locations[ 0 ]
+  else:
+    return locations
+
+
 def _LspLocationToLocationAndDescription( request_data,
                                           location,
                                           range_property = 'range' ):