V3 semantic predicate gate (=>) can only appear after a predicate {...}?, not an action {...}

tool/src/org/antlr/v4/parse/ANTLRLexer.g

@@ -298,7 +298,7 @@ ARG_ACTION
 //
 ACTION
 	:	NESTED_ACTION
-		('?' {$type = SEMPRED;} )?
+		(	'?' {$type = SEMPRED;}
 			(	'=>' // v3 gated sempred
 				{
 				Token t = new CommonToken(input, state.type, state.channel, state.tokenStartCharIndex, getCharIndex()-1);
@@ -308,6 +308,7 @@ ACTION
 				grammarError(ErrorType.V3_GATED_SEMPRED, t);
 				}
 			)?
+		)?
 	;
 
 // ----------------