c++: improve a couple of TMPL_ARGS_* accessor macros

  After r13-332-g88459c3965e2a2, it looks like we can safely remove the
NULL test from TMPL_ARGS_HAVE_MULTIPLE_LEVELS, which simplifies its
semantics.

And TMPL_ARGS_LEVEL should verify the level argument is sane in the
one-dimensional vector case.  This change uncovered a couple of latent
bugs: in try_class_unification, we weren't correctly copying
multidimensional targs, and in unify_pack_expansion it seems an
inequality test needs to be reversed (not sure about this latter fix,
the test was added in r8-6178-g2625472ffa519e FWIW).

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?  Also tested on cmcstl2 and range-v3.

gcc/cp/ChangeLog:

	* cp-tree.h (TMPL_ARGS_HAVE_MULTIPLE_LEVELS): Remove NULL test.
	(TMPL_ARGS_LEVEL): Assert LEVEL is 1 when
	TMPL_ARGS_HAVE_MULTIPLE_LEVELS is false.
	* pt.cc (try_class_unification): Correctly copy multidimensional
	targs.
	(unify_pack_expansion): Fix level comparison.
---
 gcc/cp/cp-tree.h |  5 +++--
 gcc/cp/pt.cc     | 12 ++++++------
 2 files changed, 9 insertions(+), 8 deletions(-)
  

On Thu, 12 May 2022, Patrick Palka wrote:

> After r13-332-g88459c3965e2a2, it looks like we can safely remove the
> NULL test from TMPL_ARGS_HAVE_MULTIPLE_LEVELS, which simplifies its
> semantics.
> 
> And TMPL_ARGS_LEVEL should verify the level argument is sane in the
> one-dimensional vector case.  This change uncovered a couple of latent
> bugs: in try_class_unification, we weren't correctly copying
> multidimensional targs, and in unify_pack_expansion it seems an
> inequality test needs to be reversed (not sure about this latter fix,
> the test was added in r8-6178-g2625472ffa519e FWIW).
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?  Also tested on cmcstl2 and range-v3.
> 
> gcc/cp/ChangeLog:
> 
> 	* cp-tree.h (TMPL_ARGS_HAVE_MULTIPLE_LEVELS): Remove NULL test.
> 	(TMPL_ARGS_LEVEL): Assert LEVEL is 1 when
> 	TMPL_ARGS_HAVE_MULTIPLE_LEVELS is false.
> 	* pt.cc (try_class_unification): Correctly copy multidimensional
> 	targs.
> 	(unify_pack_expansion): Fix level comparison.
> ---
>  gcc/cp/cp-tree.h |  5 +++--
>  gcc/cp/pt.cc     | 12 ++++++------
>  2 files changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index b6961a796af..f681d32ac93 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -3766,7 +3766,7 @@ struct GTY(()) lang_decl {
>  /* Nonzero if the template arguments is actually a vector of vectors,
>     rather than just a vector.  */
>  #define TMPL_ARGS_HAVE_MULTIPLE_LEVELS(NODE)		     \
> -  (NODE && TREE_VEC_LENGTH (NODE) && TREE_VEC_ELT (NODE, 0)  \
> +  (TREE_VEC_LENGTH (NODE) && TREE_VEC_ELT (NODE, 0)  \
>     && TREE_CODE (TREE_VEC_ELT (NODE, 0)) == TREE_VEC)
>  
>  /* The depth of a template argument vector.  When called directly by
> @@ -3783,7 +3783,8 @@ struct GTY(()) lang_decl {
>     args is level 1, not level 0.  */
>  #define TMPL_ARGS_LEVEL(ARGS, LEVEL)		\
>    (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (ARGS)	\
> -   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1) : (ARGS))
> +   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1)		\
> +   : (gcc_checking_assert (LEVEL == 1), (ARGS)))
>  
>  /* Set the LEVELth level of the template ARGS to VAL.  This macro does
>     not work with single-level argument vectors.  */
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 2c7c5f8bb5d..75b21e5c88a 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -23398,8 +23398,6 @@ static tree
>  try_class_unification (tree tparms, tree targs, tree parm, tree arg,
>  		       bool explain_p)
>  {
> -  tree copy_of_targs;
> -
>    if (!CLASSTYPE_SPECIALIZATION_OF_PRIMARY_TEMPLATE_P (arg))
>      return NULL_TREE;
>    else if (TREE_CODE (parm) == BOUND_TEMPLATE_TEMPLATE_PARM)
> @@ -23438,17 +23436,19 @@ try_class_unification (tree tparms, tree targs, tree parm, tree arg,
>       because there are two ways to unify base classes of S<0, 1, 2>
>       with S<I, I, I>.  If we kept the already deduced knowledge, we
>       would reject the possibility I=1.  */
> -  copy_of_targs = make_tree_vec (TREE_VEC_LENGTH (targs));
> +  targs = copy_template_args (targs);
> +  for (tree& targ : tree_vec_range (INNERMOST_TEMPLATE_ARGS (targs)))
> +    targ = NULL_TREE;

It occurred to me that we don't need to make a copy of (the TREE_VECs
for) the outer template arguments, so it might be better to use
copy_node manually instead of copy_template_args.  But this redundant
copying shouldn't matter much if we make sure to ggc_free it afterwards.
So here's v2 which additionally makes try_class_unification ggc_free
this copy of targs (bootstrap and regtest in progress):

-- >8 --

Subject: [PATCH] c++: improve a couple of TMPL_ARGS_* accessor macros

After r13-332-g88459c3965e2a2, it looks like we can safely remove the
NULL test from TMPL_ARGS_HAVE_MULTIPLE_LEVELS, which should make its
uses easier to reason about.

And TMPL_ARGS_LEVEL should verify the level argument is valid in the
one-dimensional vector case.  This change uncovered a couple of latent
issues: in try_class_unification, we weren't correctly copying
multidimensional targs, and in unify_pack_expansion it seems an
inequality test needs to be reversed.  This patch fixes both these
issues, and in passing makes the former function ggc_free the copy of
targs.

gcc/cp/ChangeLog:

	* cp-tree.h (TMPL_ARGS_HAVE_MULTIPLE_LEVELS): Remove NULL test.
	(TMPL_ARGS_LEVEL): Assert LEVEL is 1 when
	TMPL_ARGS_HAVE_MULTIPLE_LEVELS is false.
	* pt.cc (try_class_unification): Correctly copy multidimensional
	targs.  Free the copy of targs.
	(unify_pack_expansion): Fix level comparison.
---
 gcc/cp/cp-tree.h |  5 +++--
 gcc/cp/pt.cc     | 28 ++++++++++++++--------------
 2 files changed, 17 insertions(+), 16 deletions(-)

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index b6961a796af..c28a3311dde 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -3766,7 +3766,7 @@ struct GTY(()) lang_decl {
 /* Nonzero if the template arguments is actually a vector of vectors,
    rather than just a vector.  */
 #define TMPL_ARGS_HAVE_MULTIPLE_LEVELS(NODE)		     \
-  (NODE && TREE_VEC_LENGTH (NODE) && TREE_VEC_ELT (NODE, 0)  \
+  (TREE_VEC_LENGTH (NODE) && TREE_VEC_ELT (NODE, 0)  \
    && TREE_CODE (TREE_VEC_ELT (NODE, 0)) == TREE_VEC)
 
 /* The depth of a template argument vector.  When called directly by
@@ -3783,7 +3783,8 @@ struct GTY(()) lang_decl {
    args is level 1, not level 0.  */
 #define TMPL_ARGS_LEVEL(ARGS, LEVEL)		\
   (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (ARGS)	\
-   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1) : (ARGS))
+   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1)		\
+   : (gcc_checking_assert ((LEVEL) == 1), (ARGS)))
 
 /* Set the LEVELth level of the template ARGS to VAL.  This macro does
    not work with single-level argument vectors.  */
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 2c7c5f8bb5d..3df40f0d22f 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -23398,8 +23398,6 @@ static tree
 try_class_unification (tree tparms, tree targs, tree parm, tree arg,
 		       bool explain_p)
 {
-  tree copy_of_targs;
-
   if (!CLASSTYPE_SPECIALIZATION_OF_PRIMARY_TEMPLATE_P (arg))
     return NULL_TREE;
   else if (TREE_CODE (parm) == BOUND_TEMPLATE_TEMPLATE_PARM)
@@ -23438,21 +23436,23 @@ try_class_unification (tree tparms, tree targs, tree parm, tree arg,
      because there are two ways to unify base classes of S<0, 1, 2>
      with S<I, I, I>.  If we kept the already deduced knowledge, we
      would reject the possibility I=1.  */
-  copy_of_targs = make_tree_vec (TREE_VEC_LENGTH (targs));
+  targs = copy_template_args (targs);
+  for (tree& targ : tree_vec_range (INNERMOST_TEMPLATE_ARGS (targs)))
+    targ = NULL_TREE;
 
+  int err;
   if (TREE_CODE (parm) == BOUND_TEMPLATE_TEMPLATE_PARM)
-    {
-      if (unify_bound_ttp_args (tparms, copy_of_targs, parm, arg, explain_p))
-	return NULL_TREE;
-      return arg;
-    }
+    err = unify_bound_ttp_args (tparms, targs, parm, arg, explain_p);
+  else
+    err = unify (tparms, targs, CLASSTYPE_TI_ARGS (parm),
+		 CLASSTYPE_TI_ARGS (arg), UNIFY_ALLOW_NONE, explain_p);
 
-  /* If unification failed, we're done.  */
-  if (unify (tparms, copy_of_targs, CLASSTYPE_TI_ARGS (parm),
-	     CLASSTYPE_TI_ARGS (arg), UNIFY_ALLOW_NONE, explain_p))
-    return NULL_TREE;
+  if (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (targs))
+    for (tree level : tree_vec_range (targs))
+      ggc_free (level);
+  ggc_free (targs);
 
-  return arg;
+  return err ? NULL_TREE : arg;
 }
 
 /* Given a template type PARM and a class type ARG, find the unique
@@ -23649,7 +23649,7 @@ unify_pack_expansion (tree tparms, tree targs, tree packed_parms,
 
       /* Determine the index and level of this parameter pack.  */
       template_parm_level_and_index (parm_pack, &level, &idx);
-      if (level < levels)
+      if (level > levels)
 	continue;
 
       /* Keep track of the parameter packs and their corresponding
  

On 5/12/22 14:14, Patrick Palka wrote:
> On Thu, 12 May 2022, Patrick Palka wrote:
> 
>> After r13-332-g88459c3965e2a2, it looks like we can safely remove the
>> NULL test from TMPL_ARGS_HAVE_MULTIPLE_LEVELS, which simplifies its
>> semantics.
>>
>> And TMPL_ARGS_LEVEL should verify the level argument is sane in the
>> one-dimensional vector case.  This change uncovered a couple of latent
>> bugs: in try_class_unification, we weren't correctly copying
>> multidimensional targs, and in unify_pack_expansion it seems an
>> inequality test needs to be reversed (not sure about this latter fix,
>> the test was added in r8-6178-g2625472ffa519e FWIW).
>>
>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
>> trunk?  Also tested on cmcstl2 and range-v3.
>>
>> gcc/cp/ChangeLog:
>>
>> 	* cp-tree.h (TMPL_ARGS_HAVE_MULTIPLE_LEVELS): Remove NULL test.
>> 	(TMPL_ARGS_LEVEL): Assert LEVEL is 1 when
>> 	TMPL_ARGS_HAVE_MULTIPLE_LEVELS is false.
>> 	* pt.cc (try_class_unification): Correctly copy multidimensional
>> 	targs.
>> 	(unify_pack_expansion): Fix level comparison.
>> ---
>>   gcc/cp/cp-tree.h |  5 +++--
>>   gcc/cp/pt.cc     | 12 ++++++------
>>   2 files changed, 9 insertions(+), 8 deletions(-)
>>
>> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
>> index b6961a796af..f681d32ac93 100644
>> --- a/gcc/cp/cp-tree.h
>> +++ b/gcc/cp/cp-tree.h
>> @@ -3766,7 +3766,7 @@ struct GTY(()) lang_decl {
>>   /* Nonzero if the template arguments is actually a vector of vectors,
>>      rather than just a vector.  */
>>   #define TMPL_ARGS_HAVE_MULTIPLE_LEVELS(NODE)		     \
>> -  (NODE && TREE_VEC_LENGTH (NODE) && TREE_VEC_ELT (NODE, 0)  \
>> +  (TREE_VEC_LENGTH (NODE) && TREE_VEC_ELT (NODE, 0)  \
>>      && TREE_CODE (TREE_VEC_ELT (NODE, 0)) == TREE_VEC)
>>   
>>   /* The depth of a template argument vector.  When called directly by
>> @@ -3783,7 +3783,8 @@ struct GTY(()) lang_decl {
>>      args is level 1, not level 0.  */
>>   #define TMPL_ARGS_LEVEL(ARGS, LEVEL)		\
>>     (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (ARGS)	\
>> -   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1) : (ARGS))
>> +   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1)		\
>> +   : (gcc_checking_assert (LEVEL == 1), (ARGS)))
>>   
>>   /* Set the LEVELth level of the template ARGS to VAL.  This macro does
>>      not work with single-level argument vectors.  */
>> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
>> index 2c7c5f8bb5d..75b21e5c88a 100644
>> --- a/gcc/cp/pt.cc
>> +++ b/gcc/cp/pt.cc
>> @@ -23398,8 +23398,6 @@ static tree
>>   try_class_unification (tree tparms, tree targs, tree parm, tree arg,
>>   		       bool explain_p)
>>   {
>> -  tree copy_of_targs;
>> -
>>     if (!CLASSTYPE_SPECIALIZATION_OF_PRIMARY_TEMPLATE_P (arg))
>>       return NULL_TREE;
>>     else if (TREE_CODE (parm) == BOUND_TEMPLATE_TEMPLATE_PARM)
>> @@ -23438,17 +23436,19 @@ try_class_unification (tree tparms, tree targs, tree parm, tree arg,
>>        because there are two ways to unify base classes of S<0, 1, 2>
>>        with S<I, I, I>.  If we kept the already deduced knowledge, we
>>        would reject the possibility I=1.  */
>> -  copy_of_targs = make_tree_vec (TREE_VEC_LENGTH (targs));
>> +  targs = copy_template_args (targs);
>> +  for (tree& targ : tree_vec_range (INNERMOST_TEMPLATE_ARGS (targs)))
>> +    targ = NULL_TREE;
> 
> It occurred to me that we don't need to make a copy of (the TREE_VECs
> for) the outer template arguments, so it might be better to use
> copy_node manually instead of copy_template_args.  But this redundant
> copying shouldn't matter much if we make sure to ggc_free it afterwards.
> So here's v2 which additionally makes try_class_unification ggc_free
> this copy of targs (bootstrap and regtest in progress):
> 
> -- >8 --
> 
> Subject: [PATCH] c++: improve a couple of TMPL_ARGS_* accessor macros
> 
> After r13-332-g88459c3965e2a2, it looks like we can safely remove the
> NULL test from TMPL_ARGS_HAVE_MULTIPLE_LEVELS, which should make its
> uses easier to reason about.

because we should have given up before trying to check that?

> And TMPL_ARGS_LEVEL should verify the level argument is valid in the
> one-dimensional vector case.  This change uncovered a couple of latent
> issues: in try_class_unification, we weren't correctly copying
> multidimensional targs, and in unify_pack_expansion it seems an
> inequality test needs to be reversed.  This patch fixes both these
> issues, and in passing makes the former function ggc_free the copy of
> targs.

OK.

> gcc/cp/ChangeLog:
> 
> 	* cp-tree.h (TMPL_ARGS_HAVE_MULTIPLE_LEVELS): Remove NULL test.
> 	(TMPL_ARGS_LEVEL): Assert LEVEL is 1 when
> 	TMPL_ARGS_HAVE_MULTIPLE_LEVELS is false.
> 	* pt.cc (try_class_unification): Correctly copy multidimensional
> 	targs.  Free the copy of targs.
> 	(unify_pack_expansion): Fix level comparison.
> ---
>   gcc/cp/cp-tree.h |  5 +++--
>   gcc/cp/pt.cc     | 28 ++++++++++++++--------------
>   2 files changed, 17 insertions(+), 16 deletions(-)
> 
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index b6961a796af..c28a3311dde 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -3766,7 +3766,7 @@ struct GTY(()) lang_decl {
>   /* Nonzero if the template arguments is actually a vector of vectors,
>      rather than just a vector.  */
>   #define TMPL_ARGS_HAVE_MULTIPLE_LEVELS(NODE)		     \
> -  (NODE && TREE_VEC_LENGTH (NODE) && TREE_VEC_ELT (NODE, 0)  \
> +  (TREE_VEC_LENGTH (NODE) && TREE_VEC_ELT (NODE, 0)  \
>      && TREE_CODE (TREE_VEC_ELT (NODE, 0)) == TREE_VEC)
>   
>   /* The depth of a template argument vector.  When called directly by
> @@ -3783,7 +3783,8 @@ struct GTY(()) lang_decl {
>      args is level 1, not level 0.  */
>   #define TMPL_ARGS_LEVEL(ARGS, LEVEL)		\
>     (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (ARGS)	\
> -   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1) : (ARGS))
> +   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1)		\
> +   : (gcc_checking_assert ((LEVEL) == 1), (ARGS)))
>   
>   /* Set the LEVELth level of the template ARGS to VAL.  This macro does
>      not work with single-level argument vectors.  */
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 2c7c5f8bb5d..3df40f0d22f 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -23398,8 +23398,6 @@ static tree
>   try_class_unification (tree tparms, tree targs, tree parm, tree arg,
>   		       bool explain_p)
>   {
> -  tree copy_of_targs;
> -
>     if (!CLASSTYPE_SPECIALIZATION_OF_PRIMARY_TEMPLATE_P (arg))
>       return NULL_TREE;
>     else if (TREE_CODE (parm) == BOUND_TEMPLATE_TEMPLATE_PARM)
> @@ -23438,21 +23436,23 @@ try_class_unification (tree tparms, tree targs, tree parm, tree arg,
>        because there are two ways to unify base classes of S<0, 1, 2>
>        with S<I, I, I>.  If we kept the already deduced knowledge, we
>        would reject the possibility I=1.  */
> -  copy_of_targs = make_tree_vec (TREE_VEC_LENGTH (targs));
> +  targs = copy_template_args (targs);
> +  for (tree& targ : tree_vec_range (INNERMOST_TEMPLATE_ARGS (targs)))
> +    targ = NULL_TREE;
>   
> +  int err;
>     if (TREE_CODE (parm) == BOUND_TEMPLATE_TEMPLATE_PARM)
> -    {
> -      if (unify_bound_ttp_args (tparms, copy_of_targs, parm, arg, explain_p))
> -	return NULL_TREE;
> -      return arg;
> -    }
> +    err = unify_bound_ttp_args (tparms, targs, parm, arg, explain_p);
> +  else
> +    err = unify (tparms, targs, CLASSTYPE_TI_ARGS (parm),
> +		 CLASSTYPE_TI_ARGS (arg), UNIFY_ALLOW_NONE, explain_p);
>   
> -  /* If unification failed, we're done.  */
> -  if (unify (tparms, copy_of_targs, CLASSTYPE_TI_ARGS (parm),
> -	     CLASSTYPE_TI_ARGS (arg), UNIFY_ALLOW_NONE, explain_p))
> -    return NULL_TREE;
> +  if (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (targs))
> +    for (tree level : tree_vec_range (targs))
> +      ggc_free (level);
> +  ggc_free (targs);
>   
> -  return arg;
> +  return err ? NULL_TREE : arg;
>   }
>   
>   /* Given a template type PARM and a class type ARG, find the unique
> @@ -23649,7 +23649,7 @@ unify_pack_expansion (tree tparms, tree targs, tree packed_parms,
>   
>         /* Determine the index and level of this parameter pack.  */
>         template_parm_level_and_index (parm_pack, &level, &idx);
> -      if (level < levels)
> +      if (level > levels)
>   	continue;
>   
>         /* Keep track of the parameter packs and their corresponding
  

On Thu, 12 May 2022, Jason Merrill wrote:

> On 5/12/22 14:14, Patrick Palka wrote:
> > On Thu, 12 May 2022, Patrick Palka wrote:
> > 
> > > After r13-332-g88459c3965e2a2, it looks like we can safely remove the
> > > NULL test from TMPL_ARGS_HAVE_MULTIPLE_LEVELS, which simplifies its
> > > semantics.
> > > 
> > > And TMPL_ARGS_LEVEL should verify the level argument is sane in the
> > > one-dimensional vector case.  This change uncovered a couple of latent
> > > bugs: in try_class_unification, we weren't correctly copying
> > > multidimensional targs, and in unify_pack_expansion it seems an
> > > inequality test needs to be reversed (not sure about this latter fix,
> > > the test was added in r8-6178-g2625472ffa519e FWIW).
> > > 
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > > trunk?  Also tested on cmcstl2 and range-v3.
> > > 
> > > gcc/cp/ChangeLog:
> > > 
> > > 	* cp-tree.h (TMPL_ARGS_HAVE_MULTIPLE_LEVELS): Remove NULL test.
> > > 	(TMPL_ARGS_LEVEL): Assert LEVEL is 1 when
> > > 	TMPL_ARGS_HAVE_MULTIPLE_LEVELS is false.
> > > 	* pt.cc (try_class_unification): Correctly copy multidimensional
> > > 	targs.
> > > 	(unify_pack_expansion): Fix level comparison.
> > > ---
> > >   gcc/cp/cp-tree.h |  5 +++--
> > >   gcc/cp/pt.cc     | 12 ++++++------
> > >   2 files changed, 9 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> > > index b6961a796af..f681d32ac93 100644
> > > --- a/gcc/cp/cp-tree.h
> > > +++ b/gcc/cp/cp-tree.h
> > > @@ -3766,7 +3766,7 @@ struct GTY(()) lang_decl {
> > >   /* Nonzero if the template arguments is actually a vector of vectors,
> > >      rather than just a vector.  */
> > >   #define TMPL_ARGS_HAVE_MULTIPLE_LEVELS(NODE)		     \
> > > -  (NODE && TREE_VEC_LENGTH (NODE) && TREE_VEC_ELT (NODE, 0)  \
> > > +  (TREE_VEC_LENGTH (NODE) && TREE_VEC_ELT (NODE, 0)  \
> > >      && TREE_CODE (TREE_VEC_ELT (NODE, 0)) == TREE_VEC)
> > >     /* The depth of a template argument vector.  When called directly by
> > > @@ -3783,7 +3783,8 @@ struct GTY(()) lang_decl {
> > >      args is level 1, not level 0.  */
> > >   #define TMPL_ARGS_LEVEL(ARGS, LEVEL)		\
> > >     (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (ARGS)	\
> > > -   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1) : (ARGS))
> > > +   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1)		\
> > > +   : (gcc_checking_assert (LEVEL == 1), (ARGS)))
> > >     /* Set the LEVELth level of the template ARGS to VAL.  This macro does
> > >      not work with single-level argument vectors.  */
> > > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > > index 2c7c5f8bb5d..75b21e5c88a 100644
> > > --- a/gcc/cp/pt.cc
> > > +++ b/gcc/cp/pt.cc
> > > @@ -23398,8 +23398,6 @@ static tree
> > >   try_class_unification (tree tparms, tree targs, tree parm, tree arg,
> > >   		       bool explain_p)
> > >   {
> > > -  tree copy_of_targs;
> > > -
> > >     if (!CLASSTYPE_SPECIALIZATION_OF_PRIMARY_TEMPLATE_P (arg))
> > >       return NULL_TREE;
> > >     else if (TREE_CODE (parm) == BOUND_TEMPLATE_TEMPLATE_PARM)
> > > @@ -23438,17 +23436,19 @@ try_class_unification (tree tparms, tree targs,
> > > tree parm, tree arg,
> > >        because there are two ways to unify base classes of S<0, 1, 2>
> > >        with S<I, I, I>.  If we kept the already deduced knowledge, we
> > >        would reject the possibility I=1.  */
> > > -  copy_of_targs = make_tree_vec (TREE_VEC_LENGTH (targs));
> > > +  targs = copy_template_args (targs);
> > > +  for (tree& targ : tree_vec_range (INNERMOST_TEMPLATE_ARGS (targs)))
> > > +    targ = NULL_TREE;
> > 
> > It occurred to me that we don't need to make a copy of (the TREE_VECs
> > for) the outer template arguments, so it might be better to use
> > copy_node manually instead of copy_template_args.  But this redundant
> > copying shouldn't matter much if we make sure to ggc_free it afterwards.
> > So here's v2 which additionally makes try_class_unification ggc_free
> > this copy of targs (bootstrap and regtest in progress):
> > 
> > -- >8 --
> > 
> > Subject: [PATCH] c++: improve a couple of TMPL_ARGS_* accessor macros
> > 
> > After r13-332-g88459c3965e2a2, it looks like we can safely remove the
> > NULL test from TMPL_ARGS_HAVE_MULTIPLE_LEVELS, which should make its
> > uses easier to reason about.
> 
> because we should have given up before trying to check that?

*nod* I was going to mention that in particular it makes
TMPL_ARGS_LEVEL (NULL, 1) invalid instead of yielding NULL, but on
second thought yielding NULL kind of makes sense -- in some other places
we already treat NULL args like an empty TREE_VEC, so it seems
reasonable that TMPL_ARGS_LEVEL acts like the identity function for both
NULL and an empty TREE_VEC.  So I'm not too sure anymore that removing
the NULL test from TMPL_ARGS_HAVE_MULTIPLE_LEVELS is the way to go; I'm
going to remove this part of the patch, which is a non-functional change
anyway.

> 
> > And TMPL_ARGS_LEVEL should verify the level argument is valid in the
> > one-dimensional vector case.  This change uncovered a couple of latent
> > issues: in try_class_unification, we weren't correctly copying
> > multidimensional targs, and in unify_pack_expansion it seems an
> > inequality test needs to be reversed.  This patch fixes both these
> > issues, and in passing makes the former function ggc_free the copy of
> > targs.
> 
> OK.

Thanks, here's what I ultimately committed:

-- >8 --

Subject: [PATCH] c++: tighten TMPL_ARGS_LEVEL macro

This patch makes TMPL_ARGS_LEVEL verify the level argument is valid in
the one-dimensional vector case.  Doing so uncovered a couple of latent
issues: in try_class_unification, we weren't correctly copying targs
when it's two-dimensional, and in unify_pack_expansion it seems an
inequality test needs to be reversed.  This patch fixes both issues, and
in passing makes the former function free the temporary copy of targs.

gcc/cp/ChangeLog:

	* cp-tree.h (TMPL_ARGS_LEVEL): Assert LEVEL is 1 when
	TMPL_ARGS_HAVE_MULTIPLE_LEVELS is false.
	* pt.cc (try_class_unification): Correctly copy multidimensional
	targs.  Free the copy of targs.
	(unify_pack_expansion): Fix level comparison.
---
 gcc/cp/cp-tree.h |  3 ++-
 gcc/cp/pt.cc     | 28 ++++++++++++++--------------
 2 files changed, 16 insertions(+), 15 deletions(-)

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index b6961a796af..b2df6fc0ad4 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -3783,7 +3783,8 @@ struct GTY(()) lang_decl {
    args is level 1, not level 0.  */
 #define TMPL_ARGS_LEVEL(ARGS, LEVEL)		\
   (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (ARGS)	\
-   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1) : (ARGS))
+   ? TREE_VEC_ELT (ARGS, (LEVEL) - 1)		\
+   : (gcc_checking_assert ((LEVEL) == 1), (ARGS)))
 
 /* Set the LEVELth level of the template ARGS to VAL.  This macro does
    not work with single-level argument vectors.  */
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 2c7c5f8bb5d..3df40f0d22f 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -23398,8 +23398,6 @@ static tree
 try_class_unification (tree tparms, tree targs, tree parm, tree arg,
 		       bool explain_p)
 {
-  tree copy_of_targs;
-
   if (!CLASSTYPE_SPECIALIZATION_OF_PRIMARY_TEMPLATE_P (arg))
     return NULL_TREE;
   else if (TREE_CODE (parm) == BOUND_TEMPLATE_TEMPLATE_PARM)
@@ -23438,21 +23436,23 @@ try_class_unification (tree tparms, tree targs, tree parm, tree arg,
      because there are two ways to unify base classes of S<0, 1, 2>
      with S<I, I, I>.  If we kept the already deduced knowledge, we
      would reject the possibility I=1.  */
-  copy_of_targs = make_tree_vec (TREE_VEC_LENGTH (targs));
+  targs = copy_template_args (targs);
+  for (tree& targ : tree_vec_range (INNERMOST_TEMPLATE_ARGS (targs)))
+    targ = NULL_TREE;
 
+  int err;
   if (TREE_CODE (parm) == BOUND_TEMPLATE_TEMPLATE_PARM)
-    {
-      if (unify_bound_ttp_args (tparms, copy_of_targs, parm, arg, explain_p))
-	return NULL_TREE;
-      return arg;
-    }
+    err = unify_bound_ttp_args (tparms, targs, parm, arg, explain_p);
+  else
+    err = unify (tparms, targs, CLASSTYPE_TI_ARGS (parm),
+		 CLASSTYPE_TI_ARGS (arg), UNIFY_ALLOW_NONE, explain_p);
 
-  /* If unification failed, we're done.  */
-  if (unify (tparms, copy_of_targs, CLASSTYPE_TI_ARGS (parm),
-	     CLASSTYPE_TI_ARGS (arg), UNIFY_ALLOW_NONE, explain_p))
-    return NULL_TREE;
+  if (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (targs))
+    for (tree level : tree_vec_range (targs))
+      ggc_free (level);
+  ggc_free (targs);
 
-  return arg;
+  return err ? NULL_TREE : arg;
 }
 
 /* Given a template type PARM and a class type ARG, find the unique
@@ -23649,7 +23649,7 @@ unify_pack_expansion (tree tparms, tree targs, tree packed_parms,
 
       /* Determine the index and level of this parameter pack.  */
       template_parm_level_and_index (parm_pack, &level, &idx);
-      if (level < levels)
+      if (level > levels)
 	continue;
 
       /* Keep track of the parameter packs and their corresponding