report errors with too few alt labels or redef of alt label

tool/playground/A.g4

@@ -8,5 +8,11 @@ e : e '*' e 		-> Mult
   | '(' e ')'		-> Parens
   ;
 
+x : A -> Foo
+  | B
+  ;
+
+y : Y -> Mult ;
+
 INT : [0-9]+ ;
 WS : [ \t\n]+ -> skip ;

tool/playground/ABaseVisitor.java

@@ -1,10 +1,10 @@
-import org.antlr.v4.runtime.tree.*;
-import org.antlr.v4.runtime.Token;
+import org.antlr.v4.runtime.tree.ParseTreeVisitor;
 
 public class ABaseVisitor<T> extends ParseTreeVisitor<T> implements AVisitor<T> {
 	public T visit(AParser.MultContext ctx) { visitChildren(ctx); return null; }
 	public T visit(AParser.ParensContext ctx) { visitChildren(ctx); return null; }
 	public T visit(AParser.sContext ctx) { visitChildren(ctx); return null; }
+	public T visit(AParser.FooContext ctx) { visitChildren(ctx); return null; }
 	public T visit(AParser.AddContext ctx) { visitChildren(ctx); return null; }
 	public T visit(AParser.IntContext ctx) { visitChildren(ctx); return null; }
-}
+}

tool/playground/AVisitor.java

@@ -1,10 +1,8 @@
-import org.antlr.v4.runtime.tree.*;
-import org.antlr.v4.runtime.Token;
-
 public interface AVisitor<T> {
 	T visit(AParser.MultContext ctx);
 	T visit(AParser.ParensContext ctx);
 	T visit(AParser.sContext ctx);
+	T visit(AParser.FooContext ctx);
 	T visit(AParser.AddContext ctx);
 	T visit(AParser.IntContext ctx);
-}
+}

tool/src/org/antlr/v4/semantics/BasicSemanticChecks.java

@@ -40,10 +40,7 @@ import org.antlr.v4.tool.ast.*;
 import org.stringtemplate.v4.misc.MultiMap;
 
 import java.io.File;
-import java.util.ArrayList;
-import java.util.HashSet;
-import java.util.List;
-import java.util.Set;
+import java.util.*;
 
 /** No side-effects except for setting options into the appropriate node.
  *  TODO:  make the side effects into a separate pass this
@@ -242,6 +239,44 @@ public class BasicSemanticChecks extends GrammarTreeVisitor {
 //		}
 	}
 
+	Map<String,String> altLabelToRuleName = new HashMap<String, String>();
+
+	@Override
+	public void finishRule(RuleAST rule, GrammarAST ID, GrammarAST block) {
+		MultiMap<String,GrammarAST> ruleToAltLabels = new MultiMap<String, GrammarAST>();
+		if ( rule.isLexerRule() ) return;
+		BlockAST blk = (BlockAST)rule.getFirstChildWithType(BLOCK);
+		int nalts = blk.getChildCount();
+		for (int i=0; i< nalts; i++) {
+			AltAST altAST = (AltAST)blk.getChild(i);
+			if ( altAST.altLabel!=null ) {
+				System.out.println("alt label "+altAST.altLabel);
+				ruleToAltLabels.map(rule.getRuleName(), altAST.altLabel);
+				String altLabel = altAST.altLabel.getText();
+				String prevRuleForLabel = altLabelToRuleName.get(altLabel);
+				if ( prevRuleForLabel!=null ) {
+					g.tool.errMgr.grammarError(ErrorType.ALT_LABEL_REDEF,
+											   g.fileName, rule.getToken(),
+											   altLabel,
+											   rule.getRuleName(),
+											   prevRuleForLabel);
+				}
+				else {
+					altLabelToRuleName.put(altLabel, rule.getRuleName());
+				}
+			}
+		}
+		System.out.println(rule.getRuleName()+" has "+ nalts +" alts");
+		List<GrammarAST> altLabels = ruleToAltLabels.get(rule.getRuleName());
+		int numAltLabels = 0;
+		if ( altLabels!=null ) numAltLabels = altLabels.size();
+		System.out.println("labels="+altLabels);
+		if ( numAltLabels>0 && nalts != numAltLabels ) {
+			g.tool.errMgr.grammarError(ErrorType.RULE_WITH_TOO_FEW_ALT_LABELS,
+									   g.fileName, rule.getToken(), rule.getRuleName());
+		}
+	}
+
 	// Routines to do the actual work of checking issues with a grammar.
 	// They are triggered by the visitor methods above.
 

tool/src/org/antlr/v4/tool/ErrorType.java

@@ -139,7 +139,9 @@ public enum ErrorType {
 	ALL_OPS_NEED_SAME_ASSOC(118, "all operators of alt <arg> of left-recursive rule must have same associativity", ErrorSeverity.WARNING),
 	LEFT_RECURSION_CYCLES(119, "The following sets of rules are mutually left-recursive <arg:{c| [<c:{r|<r.name>}; separator=\", \">]}; separator=\" and \">", ErrorSeverity.ERROR),
 	MODE_NOT_IN_LEXER(120, "lexical modes are only allowed in lexer grammars", ErrorSeverity.ERROR),
-    CANNOT_FIND_ATTRIBUTE_NAME_IN_DECL(121, "cannot find an attribute name in attribute declaration", ErrorSeverity.ERROR),
+	CANNOT_FIND_ATTRIBUTE_NAME_IN_DECL(121, "cannot find an attribute name in attribute declaration", ErrorSeverity.ERROR),
+	RULE_WITH_TOO_FEW_ALT_LABELS(122, "rule <arg>: must label all alternatives or none", ErrorSeverity.ERROR),
+	ALT_LABEL_REDEF(123, "rule alt label <arg> redefined in rule <arg2>, originally in <arg3>", ErrorSeverity.ERROR),
 	/** Documentation comment is unterminated */
     //UNTERMINATED_DOC_COMMENT(, "", ErrorSeverity.ERROR),
 

tool/src/org/antlr/v4/tool/ast/RuleAST.java

@@ -34,8 +34,6 @@ import org.antlr.runtime.tree.Tree;
 import org.antlr.v4.parse.ANTLRParser;
 
 public class RuleAST extends GrammarASTWithOptions {
-
-
 	public RuleAST(GrammarAST node) {
 		super(node);
 	}
@@ -43,6 +41,17 @@ public class RuleAST extends GrammarASTWithOptions {
 	public RuleAST(Token t) { super(t); }
     public RuleAST(int type) { super(type); }
 
+	public boolean isLexerRule() {
+		String name = getRuleName();
+		return name!=null && Character.isUpperCase(name.charAt(0));
+	}
+
+	public String getRuleName() {
+		GrammarAST nameNode = (GrammarAST)getChild(0);
+		if ( nameNode!=null ) return nameNode.getText();
+		return null;
+	}
+
 	@Override
 	public Tree dupNode() { return new RuleAST(this); }