antlr/antlr4 · error · NotSupportedException

Not yet implemented.

Error message

Not yet implemented.

What it means

During ATN edge-factoring (set transition optimization), when a candidate set transition resolves through a NotSetTransition, the C# runtime cannot merge it and throws NotSupportedException('Not yet implemented.'). The equivalent Java code contains the same unimplemented branch, so hitting it means the optimizer encountered a ~ (not-set) transition shape it cannot factor.

Source

Thrown at runtime/CSharp/src/Atn/ATNDeserializer.cs:834

                IList<Transition> optimizedTransitions = new List<Transition>();
                for (int i_1 = 0; i_1 < decision.NumberOfOptimizedTransitions; i_1++)
                {
                    if (!setTransitions.Contains(i_1))
                    {
                        optimizedTransitions.Add(decision.GetOptimizedTransition(i_1));
                    }
                }
                ATNState blockEndState = decision.GetOptimizedTransition(setTransitions.MinElement).target.GetOptimizedTransition(0).target;
                IntervalSet matchSet = new IntervalSet();
                for (int i_2 = 0; i_2 < setTransitions.GetIntervals().Count; i_2++)
                {
                    Interval interval = setTransitions.GetIntervals()[i_2];
                    for (int j = interval.a; j <= interval.b; j++)
                    {
                        Transition matchTransition = decision.GetOptimizedTransition(j).target.GetOptimizedTransition(0);
                        if (matchTransition is NotSetTransition)
                        {
                            throw new NotSupportedException("Not yet implemented.");
                        }
                        else
                        {
                            matchSet.AddAll(matchTransition.Label);
                        }
                    }
                }
                Transition newTransition;
                if (matchSet.GetIntervals().Count == 1)
                {
                    if (matchSet.Count == 1)
                    {
                        newTransition = new AtomTransition(blockEndState, matchSet.MinElement);
                    }
                    else
                    {
                        Interval matchInterval = matchSet.GetIntervals()[0];
                        newTransition = new RangeTransition(blockEndState, matchInterval.a, matchInterval.b);

View on GitHub (pinned to 7d5770395b)

Solutions

1. Disable ATN optimization for deserialization (options with Optimize = false) so edge factoring is skipped
2. Regenerate the grammar with the matching tool/runtime version so serialized ATN matches optimizer expectations
3. If reproducible on the current release, report upstream — the branch is a known TODO

Example fix

// before
var options = ATNDeserializationOptions.Default; // optimize on
new ATNDeserializer(options).Deserialize(data);

// after
var options = new ATNDeserializationOptions();
options.Optimize = false;
options.MakeReadOnly();
new ATNDeserializer(options).Deserialize(data);

var opts = new ATNDeserializationOptions();
opts.Optimize = false;
opts.MakeReadOnly(); // avoids the unimplemented optimizer branch

try { new ATNDeserializer(options).Deserialize(data); } catch (NotSupportedException e) { when (e.Message == "Not yet implemented.") { /* retry with Optimize=false */ } }

When it happens

Trigger: Deserializing an ATN (LexerInterpreter, ParserInterpreter, or generated code with optimization enabled) whose decision contains NotSet transitions participating in a factorable set — typically grammars using '~' token sets inside decisions, combined with edge optimization.

Common situations: Complex lexer rules with negated sets ('~[...])'; older serialized ATNs reprocessed by a runtime whose optimizer added this path; rare even with such grammars because the branch needs a specific state arrangement.

Related errors

• Invalid state number.
• The object is read only.
• Couldn't identify final state of the precedence rule prefix
• Could not deserialize ATN with version {0} (expected {1}).
• The specified transition type is not valid.

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/4f15a179bc73084b. Report an issue: GitHub.