Why only one "expected"?

[Duncan Murdoch]

We're using a GNU Bison 2.3 parser, with verbose error messages. 
However, we only see messages with a single "expected" token.  For 
example, if a user forgets to give a function name in a function call 
and types "(1,1)" they'll get the error

syntax error, unexpected ',', expecting ')'

when in fact there are lots of legal tokens at that spot:  any 
arithmetic operator would be fine, such as "(1+1)", "(1-1)", etc.

Looking through the generated code, it appears that the yysyntax_error 
function should be able to handle up to 4 expected tokens, and if there 
are more than that (as here), it should default to showing none at all. 
 But that's not what's happening.

Can anyone suggest what we might be doing wrong, or is this a Bison bug?

If anyone is interested the full source is visible at

https://svn.r-project.org/R/trunk/src/main/gram.y

and the Bison output is at

https://svn.r-project.org/R/trunk/src/main/gram.c

but I'm willing to track this down myself, if I can get a hint about 
where to start looking.

Duncan Murdoch