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

[Patrick Palka <ppalka@redhat.com>]

On Thu, 15 Dec 2022, Patrick Palka wrote:

> 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.

Here's a concrete testcase that we'd begin to reject if we used
the args from DECL_FRIEND_CONTEXT for substitution:

  // Verify we substitute the correct outer template arguments
  // when instantiating a constrained template friend.
  // { dg-do compile { target c++20 } }
  
  template<class U>
    requires __is_same(int*, U)
  void f() { };
  
  template<class T>
  struct A;
  
  template<class T>
  struct A<T*> {
    template<class U>
      requires __is_same(T, U)
    friend void f() { } // { dg-bogus "redefinition" }
  };
  
  template struct A<int*>;

> 
> > 
> > > +	}
> > >         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" }
> > > +}
> > 
> > 
>