Re: semantic error: cannot expand unknown type

["Frank Ch. Eigler" <fche at redhat dot com>]

Hi, David -

> [...] Yesterday we had this conversation:
>      1. (16:05:58) dsmith: I'm getting "semantic error: cannot expand
>         unknown type" with no indication of which variable it is
>         complaining about. Got any tips on how to track it down?
> (16:06:49) fche: not pointing out a source coordinate is a bug. find the
> message in the translator source, and make it spit out the object's
> token, like the other error messages do.
> (16:08:06) dsmith: k, will do
> 
> So, I looked into it.  That error message came from
> c_unparser::c_typename().  [...]

OK.

> rwtop_fail.stp - Stap source file that triggers the error.  The problem
> is that I used "if (xyz) return" in a probe that should have been "if
> (xyz) next".  The parser grabbed the next token and tried to figure out
> what it was returning.
> 
> 1) Is this approach correct?  Could it have been done with less impact
> or in a better manner?

Before that "declare" operation, the translator code could do a quick
local unknown-type check before calling down to c_unparser, and punt.
The c_unparser::c_typename code might assert() type validity outright.

> 2) Should this be scrapped and the parser somehow upgraded to notice
> I was using a "return" instead of "next"?

There are a couple of different shortcomings there:

- In "if (cond) next", "next" should have been optimized out (replaced
  with a null statement), since it's a side-effect-free expression
  statement.  It's related to bug #2600.

- The parser does not recognize keywords as such.  Keywords are just
  tok_identifier instances with magic names.  It would be nice if a
  list of keywords was given to the scanner, and a new tok_keyword tag
  was used to represent them.  Then this "if (cond) next" would have
  been a parse error.  This is probably a worthwhile, somewhat
  mechanical change throughout parser.h/cxx.  At least our test suite
  should be enough for the "ok" case.  Want to give it a try?

- FChE