Troubles with returns declaration on the first parser rule in an ANTLR4 grammar

Question

I am using returns for my parser rules which works for all parser rules except the first one. If the first parser rule in my grammer uses the returns declaration ANTLR4 complains as follows:

expecting ARG_ACTION while matching a rule

If I add another parser rule above which does not use "returns" ANTLR does not complain.

Here you have a grammar reduced to the problem:

grammar FirstParserRuleReturnIssue;
ID  :   ('a'..'z'|'A'..'Z'|'_') ('a'..'z'|'A'..'Z'|'0'..'9'|'_')*;
aRule returns [String s]: ID { $s = $ID.text; };

I searched to find a special role of the first rule that could explain the behaviour but did not find anything. Is it a bug? Do I miss some understanding?

Answer 1

You need to place parser rules (start with a lowercase letter) before lexer rules (start with an uppercase letter) in your grammar. After encountering a lexer rule, the [ triggers a LEXER_CHAR_SET instead of ARG_ACTION, so the token stream seen by the compiler looks like you're passing a set of characters where the return value should be.