[PATCH] c++: Variable template and template parameter pack [PR96218]

[Marek Polacek]

On Wed, Jul 29, 2020 at 05:56:56PM -0400, Jason Merrill via Gcc-patches wrote:
> On 7/16/20 11:06 AM, Marek Polacek wrote:
> > This is DR 2032 which says that the restrictions regarding template
> > parameter packs and default arguments apply to variable templates as
> > well, but we weren't detecting that.
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	DR 2032
> > 	PR c++/96218
> > 	* pt.c (check_default_tmpl_args): Also consider variable
> > 	templates.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	DR 2032
> > 	PR c++/96218
> > 	* g++.dg/cpp1y/var-templ67.C: New test.
> > ---
> >   gcc/cp/pt.c                              |  5 +++--
> >   gcc/testsuite/g++.dg/cpp1y/var-templ67.C | 16 ++++++++++++++++
> >   2 files changed, 19 insertions(+), 2 deletions(-)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ67.C
> > 
> > diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> > index 4e1c77a6bd7..b74074a092b 100644
> > --- a/gcc/cp/pt.c
> > +++ b/gcc/cp/pt.c
> > @@ -5481,14 +5481,15 @@ check_default_tmpl_args (tree decl, tree parms, bool is_primary,
> >   		       /* Don't complain about an enclosing partial
> >   			  specialization.  */
> >   		       && parm_level == parms
> > -		       && TREE_CODE (decl) == TYPE_DECL
> > +		       && (TREE_CODE (decl) == TYPE_DECL || VAR_P (decl))
> 
> Can we remove this part of the test entirely, since the enclosing if already
> excludes functions?

I'm afraid we can't -- we can still get here for function templates in C++98.
Then we error like this:

q.C:1:10: error: parameter pack ‘_Args’ must be at the end of the template parameter list
    1 | template<typename... _Args, int... _Indexes>

We could move the cxx_dialect == cxx98 check probably, but not sure if that's
any better than my fix.

Marek