[Bug c++/17413] [3.4/4.0 regression] local classes as template argument

["gdr at integrable-solutions dot net" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From gdr at integrable-solutions dot net  2004-09-12 23:15 -------
Subject: Re:  [3.4/4.0 regression] local classes as template argument

"bangerth at dealii dot org" <gcc-bugzilla@gcc.gnu.org> writes:

| This is related to the substitution-failure-is-not-an-error thingy: we 
| realize that a local class cannot be matched against a template argument, 
| so we drop the template function std::for_each from the overload list -- 
| and end up with an empty list, thus the error message. 

Indeed, the whole problem is in pt.c:check_instantiated_args().  It
contains code for proper diagnostic

          tree nt = no_linkage_check (t, /*relaxed_p=*/false);

          if (nt)
            {
              if (!(complain & tf_error))
                /*OK*/;
              else if (TYPE_ANONYMOUS_P (nt))
                error ("`%T' uses anonymous type", t);
              else
                error ("`%T' uses local type `%T'", t, nt);
              result = true;
            }

But the diagnostic is not emitted because tf_error is not set in
complain. 

| We probably need to special-case the overload resolution process to handle 
| this case: when we end up with an empty overload list but there is a 
| function that would match were it not for a local class, we should report 
| so. Whether this kind of special-casing is desirable is another matter. 

I would say it is a Suboptimal implementation of SFINAE. 
In the phase where the overload-set is constructed, indeed SFINAE
is applied -- if something is invalid, just bail out without spitting
any diagnostic.  However, once the overload-set is constructed, any
error is an error.

Notice that Comeau online really does give an informative diagnostic

   "ComeauTest.c", line 11: error: a template argument may not reference
   a local type
        std::for_each(a, a + 10, T());
        ^


-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17413