I have a simple grammar:

grammar sample;
options { output = AST; }
assignment
    : IDENT ':=' expr ';'
    ;
expr    
    : factor ('*' factor)*
    ;
factor
    : primary ('+' primary)*
    ;
primary
    : NUM
    | '(' expr ')'
    ;
IDENT : ('a'..'z')+ ;
NUM   : ('0'..'9')+ ;
WS    : (' '|'\n'|'\t'|'\r')+ {$channel=HIDDEN;} ;

Now I want to add some rewrite rules to generate an AST. From what I've read online and in the Language Patterns book, I should be able to modify the grammar like this:

assignment
    : IDENT ':=' expr ';'   -> ^(':=' IDENT expr)
    ;
expr    
    : factor ('*' factor)* -> ^('*' factor+)
    ;
factor  
    : primary ('+' primary)* -> ^('+' primary+)
    ;
primary
    : NUM
    | '(' expr ')' -> ^(expr)
    ;

But it does not work. Although it compiles fine, when I run the parser I get a RewriteEmptyStreamException error. Here's where things get weird.

If I define the pseudo tokens ADD and MULT and use them instead of the tree node literals, it works without error.

tokens { ADD; MULT; }

expr    
    : factor ('*' factor)* -> ^(MULT factor+)
    ;
factor  
    : primary ('+' primary)* -> ^(ADD primary+)
    ;

Alternatively, if I use the node suffix notation, it also appears to work fine:

expr    
    : factor ('*'^ factor)*
    ;
factor  
    : primary ('+'^ primary)*
    ;

Is this discrepancy in behavior a bug?