Grammar bug :
Identifier: [A-Za-z][A-Za-z0-9_]* ;
expr: Identifier ;

Trigger :

• Input to expr is given an illegal character e.g : "id@"

• one has overriden the recover function of the lexer. e.g. :
  `class ErrorLexer(bugLexer):
  """
  New Lexer class that overrides the bugLexer and crashes in case of lexer error
  """

  def recover(self, r):
  raise r # Raise the error for the upper level`

Analysis
LexerNoViableAltException is created with message = None
Exception is caught by ErrorStrategy that reportError with a None message
print("unknown recognition error type: " + type(e).__name__) recognizer.notifyErrorListeners(e.message, e.offendingToken, e)
Then the ConsoleErrorListener will display the syntaxError as follow:
print("line " + str(line) + ":" + str(column) + " " + msg, file=sys.stderr)
This raise an error as msg is None.

Proposal is to create the LexerNoViableAltException with a message
e.g. :
def __init__(self, lexer:Lexer, input:InputStream, startIndex:int, deadEndConfigs:ATNConfigSet): super().__init__(message=None, recognizer=lexer, input=input, ctx=None) self.startIndex = startIndex self.deadEndConfigs = deadEndConfigs self.message = self.__str__()