What could be a reason for `_localctx` being null in an antlr4 semantic predicate?

Question

I'm using list labels to gather tokens and semantic predicates to validate sequences in my parser grammar. E.g.

line
:
    (text+=WORD | text+=NUMBER)+ ((BLANK | SKIP)+ (text+=WORD | text+=NUMBER)+)+ 
    {Parser.validateContext(_localctx)}? 
    (BLANK | SKIP)*
;

where

WORD: [\u0021-\u002F\u003A-\u007E]+; // printable ASCII characters (excluding SP and numbers)
NUMBER: [\u0030-\u0039]+; // printable ASCII number characters
BLANK: '\u0020';
SKIP: '\u0020\u0020' | '\t'; // two SPs or a HT symbol

The part of Parser.validateContext used to validate the line rule would be implemented like this

private static final boolean validateContext(ParserRuleContext context) {
   //.. other contexts
   if(context instanceof LineContext) 
       return "<reference-sequence>".equals(Parser.joinTokens(((LineContext) context).text, " "));
   return false;}

where Parser.joinTokens is defined as

private static String  joinTokens(java.util.List<org.antlr.v4.runtime.Token> tokens, String delimiter) {
    StringBuilder builder = new StringBuilder();
    int i = 0, n;
    if((n = tokens.size()) == 0) return "";
    builder.append(tokens.get(0).getText());
    while(++i < n) builder.append(delimiter + tokens.get(i).getText());
    return builder.toString();}

Both are put in a @parser::members clause a the beginning of the grammar file.

---

My problem is this: sometimes the _localctx reference is null and I receive "no viable alternative" errors. These are probably caused because the failing predicate guards the respective rule and no alternative is given.

Is there a reason–potentially an error on my part–why _localctx would be null?

---

UPDATE: The answer to this question seems to suggest that semantic predicates are also called during prediction. Maybe during prediction no context is created and _localctx is set to null.

Answer 1

The semantics of _localctx in a predicate are not defined. Allowable behavior includes, but is not limited to the following (and may change during any release):

1. Failing to compile (no identifier with that name)
2. Using the wrong context object
3. Not having a context object (null)

To reference the context of the current rule from within a predicate, you need to use $ctx instead.

Note that the same applies for rule parameters, locals, and/or return values which are used in a predicate. For example, the parameter a cannot be referenced as a, but must instead be $a.