Re: [C++ Patch/RFC] PR 60608

[Jason Merrill <jason at redhat dot com>]

On 07/14/2014 12:20 PM, Paolo Carlini wrote:
> I have been looking a bit into this bug, using a reduced testcase which
> simplifies the debugging quite a bit for me (a non-variadic variant is
> ok). I cannot say to already understand all the details of the issue,
> but something which strikes me as interesting, is that the DEDUCE_CALL
> passed down by add_candidate gets lost and we end up calling
> type_unification_real from unify with DEDUCE_EXACT. If I change it to an
> unconditional DEDUCE_CALL, the testcase is accepted and the testsuite
> passes with no regressions. Is that all there is to the issue?!?

Unfortunately, that goes too far.  I think we want a new DEDUCE_PARMS 
that is like DEDUCE_EXACT but does the same transformations as 
tsubst_arg_types in this part of unify_pack_expansion:

>       /* If we had explicit template arguments, substitute them into the
>          pattern before deduction.  */
>       if (any_explicit)
>         {
>           /* Some arguments might still be unspecified or dependent.  */
>           bool dependent;
>           ++processing_template_decl;
>           dependent = any_dependent_template_arguments_p (targs);
>           if (!dependent)
>             --processing_template_decl;
>           parm = tsubst (pattern, targs,
>                          explain_p ? tf_warning_or_error : tf_none,
>                          NULL_TREE);
>           if (dependent)
>             --processing_template_decl;
>           if (parm == error_mark_node)
>             return 1;
>         }

Jason