Re: [boost] [static_assert] Bad compiler message

From: Brian McNamara (lorgon_at_[hidden])
Date: 2004-04-15 02:20:22

On Thu, Apr 15, 2004 at 10:46:23AM +0400, Vladimir Prus wrote:
> Hello,
> I've just found out that the following program:
> 
>     #include <boost/static_assert.hpp>
> 
>     int main()
>     {
>         BOOST_STATIC_ASSERT(false);
>         return 0;
>     }
> 
> compiled with gcc 3.3 produces:
> 
>    sa.cpp:6: error: invalid application of `sizeof' to an incomplete type

I'm kinda hijacking your message topic (apologies) but...

More generally, I would really like a Boost library which lets me
generate custom error messages portably.  For example, with FC++, I
spent a bit of time getting g++ to give "nice" diagnostics for certain
kinds of errors -- for instance, compiling

   // uh oh...
   cout << lambda(X,X)[ X ](1,2) << endl;   // lam2.cpp line 94

with g++-3.1.1 yields

   lambda.hpp: In function 
      `boost::fcpp::lambda_impl::LambdaThingy<
      boost::fcpp::lambda_impl::exp::CONS<
      boost::fcpp::lambda_impl::exp::lambda_var<i>,
      boost::fcpp::lambda_impl::exp::CONS<
      boost::fcpp::lambda_impl::exp::lambda_var<i>,
      boost::fcpp::lambda_impl::exp::NIL> > >
      boost::fcpp::lambda_impl::lambda(const
      boost::fcpp::lambda_impl::exp::lambda_var<i>&, const
      boost::fcpp::lambda_impl::exp::lambda_var<i>&) [with int i = 4, int
      j = 4]':
   lam2.cpp:94:   instantiated from here
   lambda.hpp:1137: no method `boost::fcpp::lambda_impl::
      YouCannotPassTheSameLambdaVarTo_lambda_MoreThanOnce<true>::go'

I am happy with the last two lines of the error message, which indicate
both the source of the problem (lam2.cpp, line 94) and the issue
(YouCannotPassTheSameLambdaVarTo_lambda_MoreThanOnce).  However I got
this working using completely ad-hoc techniques which maybe only work
well for g++-3.1.1.

What I would really love is if there were a boost library which enabled
me to dispatch messages along the lines of

   BOOST_CUSTOM_ERROR( b, msg )
   BOOST_CUSTOM_WARNING( b, msg )

where in both cases "b" is a compile-time boolean, "msg" is the name of
a type, and the behavior is

   if the boolean is true
      force this compiler to generate a short diagnostic with typename
      "msg" featured prominently

In the long run, there really needs to be language support for
meta-programs to generate legible diagnostics, but in the meantime, I
would love a stopgap solution along the lines above.  I think it would
"only" involve the work of finding the best "trick" for each compiler
brand/version.  Perhaps a little concerted effort would discover one
trick which worked well across many compilers.

Is there interest in this?

-- 
-Brian McNamara (lorgon_at_[hidden])