[antlr-interest] Bug report: Unexplainable "no viable alternative"

[alex.marin at amiq.ro]

Hello,

We have trouble understanding why we get a "no viable alternative" when
running the attached parser grammar on the following input:

bit bitstream [];

The output is:

line 1:15 no viable alternative at input ']'

However, we have found two (very strange) workarounds for the issue:
1. Commenting out the 'real' option in the data_type rule
2. Using associative_dimension_2 rule instead of associative_dimension_1
(although the two are equivalent)

What is the explanation for this behavior?
Is there a rigurous solution to avoid such behavior?

Thanks,
Alex Marin

Notes:
- the example is not intended to be useful by itself (it is an excerpt from
a much larger grammar)
- the latest antlr version has been used for code generation
(antlr-3.2.jar)
- you can find the referred grammar inline at the end of this e-mail and
also in the attached file
- by comparing the generated parsers, we noticed that the workarounds cause
the prediction to be done by some complicated if-conditions rather than the
dfa which throws the NoViableAlt

////////////////// Example.g ////////////////////////

grammar Example;

options {
	k=1;
	output=AST;
	}

entry
:
(my_rule)+
;

my_rule
	:
	  tf_port_item SEMI
	;

tf_port_item
: 
data_type ID variable_dimension
;


data_type
:
'bit'
| 'byte'
| 'real' // Comment this to suppress NoViableAlt
| 'struct' 
| 'union' ( 'tagged' )? 
| 'enum' 
| 'virtual'
| ps_identifier 
;

ps_identifier
:
( ID COLON_COLON ) => ID COLON_COLON ID 
| ID
;
	
variable_dimension
:
( associative_dimension_1 ) => associative_dimension_1 variable_dimension  
// comment this line
//        ( associative_dimension_2 ) => associative_dimension_2
variable_dimension  // and uncomment this one to suppress NoViableAlt (
with 'real' alt in data_type)
| ( sized_or_unsized_dimension )*
;

associative_dimension_1
:
LBRACK ( STAR | data_type ) RBRACK
;      

associative_dimension_2
	    :
( LBRACK STAR ) => LBRACK STAR RBRACK
| LBRACK data_type RBRACK
	    ;


sized_or_unsized_dimension 
:
LBRACK ( NUMBER )? RBRACK 
;

/********** Lexer *************/

SEMI: ';';
STAR: '*';
LBRACK: '[';
RBRACK: ']';
COLON_COLON: '::';

WS
:
(' '|'\r'|'\t'|'\u000C'|'\n') {$channel=HIDDEN;}
;

ID
:
('a'..'z'|'A'..'Z'|'_') ('0'..'9'|'a'..'z'|'A'..'Z'|'_')*
;

NUMBER
:
('0'..'9')+
;
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