add {} in primary alt block to prevent ID|INT from becoming SET, which breaks code gen needs.

tool/playground/T.g

@@ -1,6 +1,8 @@
 grammar T;
-s : f f EOF;
+s : e ';' ;
-f : | x;
+e : e '*' e
-x : 'a' 'b';
+  | ID
+  | INT
+  ;
 INT : '0'..'9'+;
 WS : (' '|'\n') {skip();} ;

tool/resources/org/antlr/v4/tool/templates/LeftRecursiveRules.stg

@@ -37,7 +37,7 @@ recRule(ruleName, precArgDef, argName, primaryAlts, opAlts, setResultAction,
         userRetvals, leftRecursiveRuleRefLabels) ::=
 <<
 <ruleName>[<precArgDef>]<if(userRetvals)> returns [<userRetvals>]<endif>
-    :   ( <primaryAlts:{alt | <alt.altText> }; separator="\n        | ">
+    :   ( {} <primaryAlts:{alt | <alt.altText> }; separator="\n        | ">
         )
         ( <opAlts; separator="\n        | ">
         )*

tool/test/org/antlr/v4/test/TestLeftRecursion.java

@@ -276,6 +276,59 @@ public class TestLeftRecursion extends BaseTest {
 		runTests(grammar, tests, "s");
 	}
 
+	@Test
+	public void testAmbigLR() throws Exception {
+		String grammar =
+			"// START: g\n" +
+			"grammar Expr;\n" +
+			"// END: g\n" +
+			"\n" +
+			"// START:stat\n" +
+			"prog:   stat ;\n" +
+			"\n" +
+			"stat:   expr NEWLINE                -> printExpr\n" +
+			"    |   ID '=' expr NEWLINE         -> assign\n" +
+			"    |   NEWLINE                     -> blank\n" +
+			"    ;\n" +
+			"// END:stat\n" +
+			"\n" +
+			"// START:expr\n" +
+			"expr:   expr ('*'|'/') expr      -> MulDiv\n" +
+			"    |   expr ('+'|'-') expr      -> AddSub\n" +
+			"    |   INT                      -> int\n" +
+			"    |   ID                       -> id\n" +
+			"    |   '(' expr ')'             -> parens\n" +
+			"    ;\n" +
+			"// END:expr\n" +
+			"\n" +
+			"// show marginal cost of adding a clear/wipe command for memory\n" +
+			"\n" +
+			"// START:tokens\n" +
+			"MUL :   '*' ; // assigns token name to '*' used above in grammar\n" +
+			"DIV :   '/' ;\n" +
+			"ADD :   '+' ;\n" +
+			"SUB :   '-' ;\n" +
+			"ID  :   [a-zA-Z]+ ;      // match identifiers\n" +
+			"INT :   [0-9]+ ;         // match integers\n" +
+			"NEWLINE:'\\r'? '\\n' ;     // return newlines to parser (is end-statement signal)\n" +
+			"WS  :   [ \\t]+ -> skip ; // toss out whitespace\n" +
+			"// END:tokens\n";
+		String result = execParser("Expr.g4", grammar, "ExprParser", "ExprLexer", "prog", "1\n", true);
+		assertNull(stderrDuringParse);
+
+		result = execParser("Expr.g4", grammar, "ExprParser", "ExprLexer", "prog", "a = 5\n", true);
+		assertNull(stderrDuringParse);
+
+		result = execParser("Expr.g4", grammar, "ExprParser", "ExprLexer", "prog", "b = 6\n", true);
+		assertNull(stderrDuringParse);
+
+		result = execParser("Expr.g4", grammar, "ExprParser", "ExprLexer", "prog", "a+b*2\n", true);
+		assertNull(stderrDuringParse);
+
+		result = execParser("Expr.g4", grammar, "ExprParser", "ExprLexer", "prog", "(1+2)*3\n", true);
+		assertNull(stderrDuringParse);
+	}
+
 	public void runTests(String grammar, String[] tests, String startRule) {
 		rawGenerateAndBuildRecognizer("T.g", grammar, "TParser", "TLexer");
 		writeRecognizerAndCompile("TParser",