Null pointer in named alternative with attributes

[oldwo]

Probably the same as #822 , this one is stopping my work.

grammar probz;
statement
locals[Scope scope]
    : expressionA[$scope] ';'
    ;
expressionA[Scope scope]
    : atom[$scope]
    | expressionA[$scope] '[' expressionA[$scope] ']'
    ;
atom[Scope scope]
    : 'dummy'
    ;

Which throws an exception upon compilation:
C:\ANTLRprobz>java -jar antlr4-4.5.1-complete.jar probz.g4
new Rule() name=T__0 ast=RULE
new Rule() name=T__1 ast=RULE
new Rule() name=T__2 ast=RULE
new Rule() name=T__3 ast=RULE
getAltLabels() index=0 numberOfAlts=1 name=T__0 alt[i1].Num1.ast=ALT=>true
getAltLabels() index=1 numberOfAlts=1 name=T__1 alt[i1].Num1.ast=ALT=>true
getAltLabels() index=2 numberOfAlts=1 name=T__2 alt[i1].Num1.ast=ALT=>true
getAltLabels() index=3 numberOfAlts=1 name=T__3 alt[i1].Num1.ast=ALT=>true
new Rule() name=statement ast=RULE
new Rule() name=expressionA ast=RULE
new Rule() name=atom ast=RULE
getAltLabels() index=0 numberOfAlts=1 name=statement alt[i1].Num1.ast=ALT=>true
getAltLabels() index=1 numberOfAlts=1 name=expressionA alt[i1].Num1.ast=ALT=>true
getAltLabels() index=1 numberOfAlts=1 name=expressionA alt[i1].Num1.ast=ALT=>true
getAltLabels() index=2 numberOfAlts=1 name=atom alt[i1].Num1.ast=null=>true
context [/report INTERNAL_ERROR] 1:17 attribute arg isn't defined
error(20): internal error:
Exception in thread "main" java.lang.NullPointerException
at org.antlr.v4.semantics.SemanticPipeline.process(SemanticPipeline.java:116)
at org.antlr.v4.Tool.processNonCombinedGrammar(Tool.java:399)
at org.antlr.v4.Tool.process(Tool.java:386)
at org.antlr.v4.Tool.processGrammarsOnCommandLine(Tool.java:345)
at org.antlr.v4.Tool.main(Tool.java:192)

[oldwo]

Maybe this will help: I chose getAltLabels() as the pivot for debug prints. After encountering the first named alternative (there were many initially) all following alternatives have member .ast as null. Apparently, because ANTLR fails to discover the outer altrnative:

    public void discoverOuterAlt(AltAST alt) {
        currentRule.alt[currentOuterAltNumber].ast = alt;
    }

in SymbolCollector.java

[ericvergnaud]

afaik, direct left recursion with parameters is not supported, see:

http://stackoverflow.com/questions/18523592/antlr-4-can-not-compile-direct-recursive-expressions-with-parameters

however, the tool should complain, not crash...

[samwalls]

I am getting the same errors including and following "...INTERNAL_ERROR] 1:17 attribute arg is not defined", but I am not using parameters at all. I really wish that error description was better, is there a way I can track the building process to find out precisely what is causing issues?

[parrt]

That arg is in the ST templates during code gen. It's "internal" ;)

[samwalls]

ok then, is this actually a bug; or is there something I can do?

[parrt]

I'm thinking bug or illegal with bad error message

[samwalls]

hm, this is slightly unrelated I suppose then, where would you recommend that I get help identifying if it is my grammar that is causing the issue? I know that posting on stack overflow is discouraged for ANTLR these days.

[parrt]

Confirmed. I get:

maniac:/tmp $ a4 probz.g4
context [/report INTERNAL_ERROR] 1:17 attribute arg isn't defined
error(20):  internal error:  
Exception in thread "main" java.lang.NullPointerException
    at org.antlr.v4.tool.Rule.getAltLabels(Rule.java:219)
    at org.antlr.v4.tool.Rule.hasAltSpecificContexts(Rule.java:202)
    at org.antlr.v4.semantics.SymbolChecks.checkForLabelConflicts(SymbolChecks.java:157)
    at org.antlr.v4.semantics.SymbolChecks.process(SymbolChecks.java:105)
    at org.antlr.v4.semantics.SemanticPipeline.process(SemanticPipeline.java:108)
    at org.antlr.v4.Tool.processNonCombinedGrammar(Tool.java:400)
    at org.antlr.v4.Tool.process(Tool.java:387)
    at org.antlr.v4.Tool.processGrammarsOnCommandLine(Tool.java:346)
    at org.antlr.v4.Tool.main(Tool.java:193)