Re: [PATCH] c++: template friend with variadic constraints [PR108066]

[Patrick Palka <ppalka@redhat.com>]

On Thu, 15 Dec 2022, Jason Merrill wrote:

> On 12/12/22 12:20, Patrick Palka wrote:
> > When instantiating a constrained hidden template friend, we need to
> > substitute into its constraints for sake of declaration matching.
> 
> Hmm, we shouldn't need to do declaration matching when there's only a single
> declaration.

Oops, indeed.  It looks like in this case we're not calling
maybe_substitute_reqs_for during declaration matching, but rather from
tsubst_friend_function and specifically for the non-trailing requirements:

  if (TREE_CODE (new_friend) == TEMPLATE_DECL)
    {
      DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P (new_friend) = false;
      DECL_USE_TEMPLATE (DECL_TEMPLATE_RESULT (new_friend)) = 0;
      DECL_SAVED_TREE (DECL_TEMPLATE_RESULT (new_friend))
        = DECL_SAVED_TREE (DECL_TEMPLATE_RESULT (decl));

      /* Substitute TEMPLATE_PARMS_CONSTRAINTS so that parameter levels will
         match in decls_match.  */
      tree parms = DECL_TEMPLATE_PARMS (new_friend);
      tree treqs = TEMPLATE_PARMS_CONSTRAINTS (parms);
      treqs = maybe_substitute_reqs_for (treqs, new_friend);
      if (treqs != TEMPLATE_PARMS_CONSTRAINTS (parms))
        {
          TEMPLATE_PARMS_CONSTRAINTS (parms) = treqs;
          /* As well as each TEMPLATE_PARM_CONSTRAINTS.  */
          tsubst_each_template_parm_constraints (parms, args,
                                                 tf_warning_or_error);
        }
    }

I'll adjust the commit message appropriately.

> 
> > For
> > this substitution we use a full argument vector whose outer levels
> > correspond to the class's arguments and innermost level corresponds to
> > the template's level-lowered generic arguments.
> > 
> > But for A<int>::f here, for which the relevant argument vector is
> > {{int}, {Us...}}, this substitution triggers the assert in
> > use_pack_expansion_extra_args_p since one argument is a pack expansion
> > and the other isn't.
> > 
> > And for A<int, int>::f, for which the relevant argument vector is
> > {{int, int}, {Us...}}, the use_pack_expansion_extra_args_p assert would
> > also trigger but we first get a bogus "mismatched argument pack lengths"
> > error from tsubst_pack_expansion.
> > 
> > These might ultimately be bugs in tsubst_pack_expansion, but it seems
> > we can work around them by tweaking the constraint substitution in
> > maybe_substitute_reqs_for to only use the friend's outer arguments in
> > the case of a template friend.
> 
> Yes, this is how we normally substitute a member template during class
> instantiation: with the class' template args, not its own.  The assert seems
> to be enforcing that.

Ah, makes snese.

> 
> > This should be otherwise equivalent to
> > substituting using the full arguments, since the template's innermost
> > arguments are just its generic arguments with level=1.
> > 
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk/12?
> > 
> > 	PR c++/108066
> > 	PR c++/108067
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* constraint.cc (maybe_substitute_reqs_for): For a template
> > 	friend, substitute using only its outer arguments.  Remove
> > 	dead uses_template_parms test.
> 
> I don't see any removal?

Oops, I reverted that change before posting the patch but forgot to adjust the
ChangeLog as well.  Removing the uses_template_parms test seems to be safe but
it's not necessary to fix the bug.

> 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/cpp2a/concepts-friend12.C: New test.
> > ---
> >   gcc/cp/constraint.cc                          |  8 +++++++
> >   .../g++.dg/cpp2a/concepts-friend12.C          | 22 +++++++++++++++++++
> >   2 files changed, 30 insertions(+)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-friend12.C
> > 
> > diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> > index d4cd92ec3b4..f9d1009c9fe 100644
> > --- a/gcc/cp/constraint.cc
> > +++ b/gcc/cp/constraint.cc
> > @@ -1352,6 +1352,14 @@ maybe_substitute_reqs_for (tree reqs, const_tree
> > decl)
> >         tree tmpl = DECL_TI_TEMPLATE (decl);
> >         tree gargs = generic_targs_for (tmpl);
> >         processing_template_decl_sentinel s;
> > +      if (PRIMARY_TEMPLATE_P (tmpl))
> > +	{
> > +	  if (TEMPLATE_ARGS_DEPTH (gargs) == 1)
> > +	    return reqs;
> > +	  ++processing_template_decl;
> > +	  gargs = copy_node (gargs);
> > +	  --TREE_VEC_LENGTH (gargs);
> 
> Maybe instead of messing with TEMPLATE_ARGS_DEPTH we want to grab the targs
> for DECL_FRIEND_CONTEXT instead of decl itself?

IIUC DECL_FRIEND_CONTEXT wouldn't give us the right args if the template friend
is declared inside a partial specialization:

  template<class T, class U>
  concept C = __is_same(T, U);

  template<class T>
  struct A;

  template<class T>
  struct A<T*> {
    template<class U>
      requires (__is_same(T, U))
    friend void f() { };
  };

  template struct A<int*>;

The DECL_FRIEND_CONTEXT of f is A<int*>, whose TYPE_TI_ARGS are {int*},
relative to the primary template instead of the partial specialization.  So
we'd wrongly end up substituting T=int* instead of T=int into f's
TEMPLATE_PARMS_CONSTRAINTS.

r12-6950-g76dc465aaf1b74 fixed a similar issue when substituting into
class-scope deduction guides declared inside a partial specialization.

> 
> > +	}
> >         if (uses_template_parms (gargs))
> >   	++processing_template_decl;
> >         reqs = tsubst_constraint (reqs, gargs,
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-friend12.C
> > b/gcc/testsuite/g++.dg/cpp2a/concepts-friend12.C
> > new file mode 100644
> > index 00000000000..95973842afb
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend12.C
> > @@ -0,0 +1,22 @@
> > +// PR c++/108066
> > +// PR c++/108067
> > +// { dg-do compile { target c++20 } }
> > +
> > +template<class T, class U>
> > +concept C = __is_same(T, U);
> > +
> > +template<class... Ts>
> > +struct A {
> > +  template<class... Us>
> > +    requires (... && C<Ts, Us>)
> > +  friend void f(A, A<Us...>) { }
> > +};
> > +
> > +int main() {
> > +  A<int> x;
> > +  f(x, x);
> > +  A<int, int> y;
> > +  f(y, y);
> > +  A<char> z;
> > +  f(x, z); // { dg-error "no match" }
> > +}
> 
>