Re: [PATCH 1/2] c++: improve "NTTP argument considered unused" fix [PR53164, PR105848]

[Patrick Palka <ppalka@redhat.com>]

On Thu, 30 Mar 2023, Jason Merrill wrote:

> On 3/30/23 14:53, Patrick Palka wrote:
> > On Wed, 29 Mar 2023, Jason Merrill wrote:
> > 
> > > On 3/27/23 09:30, Patrick Palka wrote:
> > > > On Thu, 23 Mar 2023, Patrick Palka wrote:
> > > > 
> > > > > r13-995-g733a792a2b2e16 worked around the problem of FUNCTION_DECL
> > > > > template arguments not always getting marked as odr-used by
> > > > > redundantly
> > > > > calling mark_used on the substituted ADDR_EXPR callee of a CALL_EXPR.
> > > > > This is just a narrow workaround however, since using a FUNCTION_DECL
> > > > > as
> > > > > a template argument alone should constitutes an odr-use; we shouldn't
> > > > > need to subsequently e.g. call the function or take its address.
> > > 
> > > Agreed.  But why didn't we already wrap it in an ADDR_EXPR?  Even for
> > > reference tparms convert_nontype_argument should do that.
> > 
> > Indeed we do, the commit message was just rather sloppy/inaccurate...
> > I'll try to correct it.
> > 
> > > 
> > > > > This patch fixes this in a more general way at template specialization
> > > > > time by walking the template arguments of the specialization and
> > > > > calling
> > > > > mark_used on all entities used within.  As before, the call to
> > > > > mark_used
> > > > > as it worst a no-op, but it compensates for the situation where we end
> > > > > up
> > > > > forming a specialization from a template context in which mark_used is
> > > > > inhibited.  Another approach would be to call mark_used whenever we
> > > > > substitute a TEMPLATE_PARM_INDEX, but that would result in many more
> > > > > redundant calls to mark_used compared to this approach.
> > > > > 
> > > > > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
> > > > > for
> > > > > trunk?
> > > > > 
> > > > > 	PR c++/53164
> > > > > 	PR c++/105848
> > > > > 
> > > > > gcc/cp/ChangeLog:
> > > > > 
> > > > > 	* pt.cc (instantiate_class_template): Call
> > > > > 	mark_template_arguments_used.
> > > > > 	(tsubst_copy_and_build) <case CALL_EXPR>: Revert r13-995
> > > > > change.
> > > > > 	(mark_template_arguments_used): Define.
> > > > > 	(instantiate_template): Call mark_template_arguments_used.
> > > > > 
> > > > > gcc/testsuite/ChangeLog:
> > > > > 
> > > > > 	* g++.dg/template/fn-ptr3a.C: New test.
> > > > > 	* g++.dg/template/fn-ptr4.C: New test.
> > > > > ---
> > > > >    gcc/cp/pt.cc                             | 51
> > > > > ++++++++++++++++--------
> > > > >    gcc/testsuite/g++.dg/template/fn-ptr3a.C | 25 ++++++++++++
> > > > >    gcc/testsuite/g++.dg/template/fn-ptr4.C  | 14 +++++++
> > > > >    3 files changed, 74 insertions(+), 16 deletions(-)
> > > > >    create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr3a.C
> > > > >    create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr4.C
> > > > > 
> > > > > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > > > > index 7e4a8de0c8b..9b3cc33331c 100644
> > > > > --- a/gcc/cp/pt.cc
> > > > > +++ b/gcc/cp/pt.cc
> > > > > @@ -220,6 +220,7 @@ static tree make_argument_pack (tree);
> > > > >    static tree enclosing_instantiation_of (tree tctx);
> > > > >    static void instantiate_body (tree pattern, tree args, tree d, bool
> > > > > nested);
> > > > >    static tree maybe_dependent_member_ref (tree, tree, tsubst_flags_t,
> > > > > tree);
> > > > > +static void mark_template_arguments_used (tree);
> > > > >      /* Make the current scope suitable for access checking when we
> > > > > are
> > > > >       processing T.  T can be FUNCTION_DECL for instantiated function
> > > > > @@ -12142,6 +12143,9 @@ instantiate_class_template (tree type)
> > > > >          cp_unevaluated_operand = 0;
> > > > >          c_inhibit_evaluation_warnings = 0;
> > > > >        }
> > > > > +
> > > > > +  mark_template_arguments_used (INNERMOST_TEMPLATE_ARGS (args));
> > > > > +
> > > > >      /* Use #pragma pack from the template context.  */
> > > > >      saved_maximum_field_alignment = maximum_field_alignment;
> > > > >      maximum_field_alignment = TYPE_PRECISION (pattern);
> > > > > @@ -21173,22 +21177,10 @@ tsubst_copy_and_build (tree t,
> > > > >    	  }
> > > > >      	/* Remember that there was a reference to this entity.  */
> > > > > -	if (function != NULL_TREE)
> > > > > -	  {
> > > > > -	    tree inner = function;
> > > > > -	    if (TREE_CODE (inner) == ADDR_EXPR
> > > > > -		&& TREE_CODE (TREE_OPERAND (inner, 0)) ==
> > > > > FUNCTION_DECL)
> > > > > -	      /* We should already have called mark_used when taking
> > > > > the
> > > > > -		 address of this function, but do so again anyway to
> > > > > make
> > > > > -		 sure it's odr-used: at worst this is a no-op, but if
> > > > > we
> > > > > -		 obtained this FUNCTION_DECL as part of ahead-of-time
> > > > > overload
> > > > > -		 resolution then that call to mark_used wouldn't have
> > > > > marked
> > > > > it
> > > > > -		 odr-used yet (53164).  */
> > > > > -	      inner = TREE_OPERAND (inner, 0);
> > > > > -	    if (DECL_P (inner)
> > > > > -		&& !mark_used (inner, complain) && !(complain &
> > > > > tf_error))
> > > > > -	      RETURN (error_mark_node);
> > > > > -	  }
> > > > > +	if (function != NULL_TREE
> > > > > +	    && DECL_P (function)
> > > > > +	    && !mark_used (function, complain) && !(complain &
> > > > > tf_error))
> > > > > +	  RETURN (error_mark_node);
> > > > >      	if (!maybe_fold_fn_template_args (function, complain))
> > > > >    	  return error_mark_node;
> > > > > @@ -21883,6 +21875,31 @@ check_instantiated_args (tree tmpl, tree
> > > > > args,
> > > > > tsubst_flags_t complain)
> > > > >      return result;
> > > > >    }
> > > > >    +/* Call mark_used on each entity within the template arguments
> > > > > ARGS of
> > > > > some
> > > > > +   template specialization, to ensure that each such entity is
> > > > > considered
> > > > > +   odr-used regardless of whether the specialization was first formed
> > > > > in
> > > > > a
> > > > > +   template context.
> > > > > +
> > > > > +   This function assumes push_to_top_level has been called
> > > > > beforehand,
> > > > > and
> > > > > +   that processing_template_decl has been set iff the template
> > > > > arguments
> > > > > +   are dependent.  */
> > > > > +
> > > > > +static void
> > > > > +mark_template_arguments_used (tree args)
> > > > > +{
> > > > > +  gcc_checking_assert (TMPL_ARGS_DEPTH (args) == 1);
> > > > > +
> > > > > +  if (processing_template_decl)
> > > > > +    return;
> > > > > +
> > > > > +  auto mark_used_r = [](tree *tp, int *, void *) {
> > > > > +    if (DECL_P (*tp))
> > > > > +      mark_used (*tp, tf_none);
> > > > > +    return NULL_TREE;
> > > > > +  };
> > > > > +  cp_walk_tree_without_duplicates (&args, mark_used_r, nullptr);
> > > > 
> > > > Here's v2 which optimizes this function by not walking if
> > > > !PRIMARY_TEMPLATE_P (since in that case the innermost arguments have
> > > > already been walked when instantiating the context), and walking only
> > > > non-type arguments:
> > > > 
> > > > -- >8
> > > > 
> > > > Subject: [PATCH] c++: improve "NTTP argument considered unused" fix
> > > > [PR53164,
> > > >    PR105848]
> > > > 
> > > > 	PR c++/53164
> > > > 	PR c++/105848
> > > > 
> > > > gcc/cp/ChangeLog:
> > > > 
> > > > 	* pt.cc (instantiate_class_template): Call
> > > > 	mark_template_arguments_used.
> > > > 	(tsubst_copy_and_build) <case CALL_EXPR>: Revert r13-995 change.
> > > > 	(mark_template_arguments_used): Define.
> > > > 	(instantiate_template): Call mark_template_arguments_used.
> > > > 
> > > > gcc/testsuite/ChangeLog:
> > > > 
> > > > 	* g++.dg/template/fn-ptr3a.C: New test.
> > > > 	* g++.dg/template/fn-ptr4.C: New test.
> > > > ---
> > > >    gcc/cp/pt.cc                             | 57
> > > > +++++++++++++++++-------
> > > >    gcc/testsuite/g++.dg/template/fn-ptr3a.C | 25 +++++++++++
> > > >    gcc/testsuite/g++.dg/template/fn-ptr4.C  | 14 ++++++
> > > >    3 files changed, 80 insertions(+), 16 deletions(-)
> > > >    create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr3a.C
> > > >    create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr4.C
> > > > 
> > > > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > > > index 3bb98ebeac1..a87366bb616 100644
> > > > --- a/gcc/cp/pt.cc
> > > > +++ b/gcc/cp/pt.cc
> > > > @@ -220,6 +220,7 @@ static tree make_argument_pack (tree);
> > > >    static tree enclosing_instantiation_of (tree tctx);
> > > >    static void instantiate_body (tree pattern, tree args, tree d, bool
> > > > nested);
> > > >    static tree maybe_dependent_member_ref (tree, tree, tsubst_flags_t,
> > > > tree);
> > > > +static void mark_template_arguments_used (tree, tree);
> > > >      /* Make the current scope suitable for access checking when we are
> > > >       processing T.  T can be FUNCTION_DECL for instantiated function
> > > > @@ -12152,6 +12153,9 @@ instantiate_class_template (tree type)
> > > >          cp_unevaluated_operand = 0;
> > > >          c_inhibit_evaluation_warnings = 0;
> > > >        }
> > > > +
> > > > +  mark_template_arguments_used (templ, CLASSTYPE_TI_ARGS (type));
> > > > +
> > > >      /* Use #pragma pack from the template context.  */
> > > >      saved_maximum_field_alignment = maximum_field_alignment;
> > > >      maximum_field_alignment = TYPE_PRECISION (pattern);
> > > > @@ -21192,22 +21196,10 @@ tsubst_copy_and_build (tree t,
> > > >    	  }
> > > >      	/* Remember that there was a reference to this entity.  */
> > > > -	if (function != NULL_TREE)
> > > > -	  {
> > > > -	    tree inner = function;
> > > > -	    if (TREE_CODE (inner) == ADDR_EXPR
> > > > -		&& TREE_CODE (TREE_OPERAND (inner, 0)) == FUNCTION_DECL)
> > > > -	      /* We should already have called mark_used when taking the
> > > > -		 address of this function, but do so again anyway to make
> > > > -		 sure it's odr-used: at worst this is a no-op, but if we
> > > > -		 obtained this FUNCTION_DECL as part of ahead-of-time overload
> > > > -		 resolution then that call to mark_used wouldn't have marked
> > > > it
> > > > -		 odr-used yet (53164).  */
> > > > -	      inner = TREE_OPERAND (inner, 0);
> > > > -	    if (DECL_P (inner)
> > > > -		&& !mark_used (inner, complain) && !(complain & tf_error))
> > > > -	      RETURN (error_mark_node);
> > > > -	  }
> > > > +	if (function != NULL_TREE
> > > > +	    && DECL_P (function)
> > > > +	    && !mark_used (function, complain) && !(complain & tf_error))
> > > > +	  RETURN (error_mark_node);
> > > >      	if (!maybe_fold_fn_template_args (function, complain))
> > > >    	  return error_mark_node;
> > > > @@ -21902,6 +21894,37 @@ check_instantiated_args (tree tmpl, tree args,
> > > > tsubst_flags_t complain)
> > > >      return result;
> > > >    }
> > > >    +/* Call mark_used on each entity within the non-type template
> > > > arguments
> > > > in
> > > > +   ARGS for a specialization of TMPL, to ensure that each such entity
> > > > is
> > > > +   considered odr-used regardless of whether the specialization was
> > > > first
> > > > +   formed in a template context.
> > > > +
> > > > +   This function assumes push_to_top_level has been called beforehand,
> > > > and
> > > > +   that processing_template_decl is set iff the template arguments are
> > > > +   dependent.  */
> > > > +
> > > > +static void
> > > > +mark_template_arguments_used (tree tmpl, tree args)
> > > > +{
> > > > +  /* It suffices to do this only when fully specializing a primary
> > > > template.  */
> > > > +  if (processing_template_decl || !PRIMARY_TEMPLATE_P (tmpl))
> > > > +    return;
> > > > +
> > > > +  /* We already marked outer arguments when specializing the context.
> > > > */
> > > > +  args = INNERMOST_TEMPLATE_ARGS (args);
> > > > +
> > > > +  for (tree arg : tree_vec_range (args))
> > > > +    if (!TYPE_P (arg))
> > > > +      {
> > > > +	auto mark_used_r = [](tree *tp, int *, void *) {
> > > > +	  if (DECL_P (*tp))
> > > > +	    mark_used (*tp, tf_none);
> > > > +	  return NULL_TREE;
> > > > +	};
> > > > +	cp_walk_tree_without_duplicates (&args, mark_used_r, nullptr);
> > > 
> > > Do we really need to walk into the args?
> > 
> > Hmm, I think we might need to some sort for walking for class NTTP
> > arguments since they could be nested CONSTRUCTORs.  But for non-class
> > arguments we know that an entity will only appear as a pointer- or
> > reference-to-function/variable, so we could pattern match for such a
> > structure directly.  And doing so should be preferable because
> > cp_walk_tree is relatively expensive and this is a relatively hot code
> > path.  So maybe the following would be preferable?
> > 
> > -- >8 --
> > 
> > Subject: [PATCH] c++: improve "NTTP argument considered unused" fix
> > [PR53164,
> >   PR105848]
> > 
> > r13-995-g733a792a2b2e16 worked around the problem of (pointer to)
> > function NTTP arguments not always getting marked as odr-used by
> > redundantly calling mark_used on the substituted ADDR_EXPR callee of a
> > CALL_EXPR.  That is just a narrow workaround however, since it assumes
> > the function is later called.  But the use as a template argument alone
> > should constitute an odr-use of the function (since template arguments
> > are an evaluated context, and we need its address); we shouldn't need to
> > subsequently call or otherwise use the NTTP argument.
> > 
> > This patch fixes this in a more general way at template specialization
> > time by walking the template arguments of the specialization and calling
> > mark_used on all entities used within.  As before, the call to mark_used
> > as it worst a no-op, but it compensates for the situation where we end
> > up forming a specialization in a template context in which mark_used is
> > inhibited.
> > 
> > Another approach would be to call mark_used whenever we substitute a
> > TEMPLATE_PARM_INDEX, but that would result in many more redundant calls
> > to mark_used compared to this approach.  And as the second testcase
> > below illustrates, we also need to walk C++20 class NTTP arguments which
> > can in theory be large and thus expensive to walk repeatedly.  The
> > change to invalid_tparm_referent_p is needed to avoid bogusly rejecting
> > the testcase's class NTTP arguments containing function pointers.
> > 
> > (The third testcase is unrelated to this fix, but it helped rule out an
> > earlier approach I was considering and it seems we don't have existing
> > test coverage for this situation.)
> > 
> > 	PR c++/53164
> > 	PR c++/105848
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* pt.cc (invalid_tparm_referent_p): Accept ADDR_EXPR of
> > 	FUNCTION_DECL.
> > 	(instantiate_class_template): Call mark_template_arguments_used.
> > 	(tsubst_copy_and_build) <case CALL_EXPR>: Revert r13-995 change.
> > 	(mark_template_arguments_used): Define.
> > 	(instantiate_template): Call mark_template_arguments_used.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/template/fn-ptr3a.C: New test.
> > 	* g++.dg/template/fn-ptr3b.C: New test.
> > 	* g++.dg/template/fn-ptr4.C: New test.
> > ---
> >   gcc/cp/pt.cc                             | 78 ++++++++++++++++++------
> >   gcc/testsuite/g++.dg/template/fn-ptr3a.C | 25 ++++++++
> >   gcc/testsuite/g++.dg/template/fn-ptr3b.C | 28 +++++++++
> >   gcc/testsuite/g++.dg/template/fn-ptr4.C  | 14 +++++
> >   4 files changed, 127 insertions(+), 18 deletions(-)
> >   create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr3a.C
> >   create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr3b.C
> >   create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr4.C
> > 
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index e514a277872..01ab220320c 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -220,6 +220,7 @@ static tree make_argument_pack (tree);
> >   static tree enclosing_instantiation_of (tree tctx);
> >   static void instantiate_body (tree pattern, tree args, tree d, bool
> > nested);
> >   static tree maybe_dependent_member_ref (tree, tree, tsubst_flags_t, tree);
> > +static void mark_template_arguments_used (tree, tree);
> >     /* Make the current scope suitable for access checking when we are
> >      processing T.  T can be FUNCTION_DECL for instantiated function
> > @@ -7142,12 +7143,13 @@ invalid_tparm_referent_p (tree type, tree expr,
> > tsubst_flags_t complain)
> >   	      decl = TREE_OPERAND (decl, 0);
> >   	    }
> >   -	if (!VAR_P (decl))
> > +	if (!VAR_OR_FUNCTION_DECL_P (decl))
> >   	  {
> >   	    if (complain & tf_error)
> >   	      error_at (cp_expr_loc_or_input_loc (expr),
> >   			"%qE is not a valid template argument of type %qT "
> > -			"because %qE is not a variable", expr, type, decl);
> > +			"because %qE is not a variable or function",
> > +			expr, type, decl);
> >   	    return true;
> >   	  }
> >   	else if (cxx_dialect < cxx11 && !DECL_EXTERNAL_LINKAGE_P (decl))
> > @@ -12152,6 +12154,9 @@ instantiate_class_template (tree type)
> >         cp_unevaluated_operand = 0;
> >         c_inhibit_evaluation_warnings = 0;
> >       }
> > +
> > +  mark_template_arguments_used (templ, CLASSTYPE_TI_ARGS (type));
> > +
> >     /* Use #pragma pack from the template context.  */
> >     saved_maximum_field_alignment = maximum_field_alignment;
> >     maximum_field_alignment = TYPE_PRECISION (pattern);
> > @@ -21192,22 +21197,10 @@ tsubst_copy_and_build (tree t,
> >   	  }
> >     	/* Remember that there was a reference to this entity.  */
> > -	if (function != NULL_TREE)
> > -	  {
> > -	    tree inner = function;
> > -	    if (TREE_CODE (inner) == ADDR_EXPR
> > -		&& TREE_CODE (TREE_OPERAND (inner, 0)) == FUNCTION_DECL)
> > -	      /* We should already have called mark_used when taking the
> > -		 address of this function, but do so again anyway to make
> > -		 sure it's odr-used: at worst this is a no-op, but if we
> > -		 obtained this FUNCTION_DECL as part of ahead-of-time overload
> > -		 resolution then that call to mark_used wouldn't have marked
> > it
> > -		 odr-used yet (53164).  */
> > -	      inner = TREE_OPERAND (inner, 0);
> > -	    if (DECL_P (inner)
> > -		&& !mark_used (inner, complain) && !(complain & tf_error))
> > -	      RETURN (error_mark_node);
> > -	  }
> > +	if (function != NULL_TREE
> > +	    && DECL_P (function)
> > +	    && !mark_used (function, complain) && !(complain & tf_error))
> > +	  RETURN (error_mark_node);
> >     	if (!maybe_fold_fn_template_args (function, complain))
> >   	  return error_mark_node;
> > @@ -21902,6 +21895,53 @@ check_instantiated_args (tree tmpl, tree args,
> > tsubst_flags_t complain)
> >     return result;
> >   }
> >   +/* Call mark_used on each entity within the non-type template arguments
> > in
> > +   ARGS for a specialization of TMPL, to ensure that each such entity is
> > +   considered odr-used regardless of whether the specialization was first
> > +   formed in a template context.
> > +
> > +   This function assumes push_to_top_level has been called beforehand, and
> > +   that processing_template_decl is set iff the template arguments are
> > +   dependent.  */
> > +
> > +static void
> > +mark_template_arguments_used (tree tmpl, tree args)
> > +{
> > +  /* It suffices to do this only when fully specializing a primary
> > template.  */
> > +  if (processing_template_decl || !PRIMARY_TEMPLATE_P (tmpl))
> > +    return;
> > +
> > +  /* We already marked outer arguments when specializing the context.  */
> > +  args = INNERMOST_TEMPLATE_ARGS (args);
> > +
> > +  for (tree arg : tree_vec_range (args))
> > +    {
> > +      /* A (pointer/reference to) function or variable NTTP argument.  */
> > +      if (TREE_CODE (arg) == ADDR_EXPR
> > +	  || TREE_CODE (arg) == INDIRECT_REF)
> > +	{
> > +	  while (TREE_CODE (arg) == ADDR_EXPR
> > +		 || REFERENCE_REF_P (arg)
> > +		 || CONVERT_EXPR_P (arg))
> > +	    arg = TREE_OPERAND (arg, 0);
> > +	  if (DECL_P (arg))
> > +	    mark_used (arg, tf_none);
> 
> Passing tf_none and also ignoring the return value needs a comment justifying
> why you assume it can't fail.

*nod* Since these calls to mark_used ought to be at worst redundant, it
should be safe to even assert

  bool ok = mark_used (arg, tf_none);
  gcc_checking_assert (ok || seen_error ());

> 
> > +	}
> > +      /* A class NTTP argument.  */
> > +      else if (VAR_P (arg)
> > +	       && DECL_NTTP_OBJECT_P (arg))
> 
> Maybe avoid doing this multiple times for the same NTTP object by gating it on
> DECL_ODR_USED (arg)?

Looks like these VAR_DECLs don't have DECL_LANG_SPECIFIC, so we can't
inspect DECL_ODR_USED for them..

It occurred to me that instantiate_class_template is perhaps the wrong
place to do this marking; it'd be more appropriate to do it in
lookup_template_class when forming the type specialization rather than
when instantiating its definition.  So in order to handle all template
kinds consistently, I think we should do the marking in either
instantiate_class_template and instantiate_body (i.e. at instantiation
time), or in lookup_template_class and instantiate_template (i.e. at
specialization time), but currently the patch does it in
instantiate_class_template and instantiate_template which sounds
consistent but it's sadly not.

However, lookup_template_class doesn't seem to call push_to_top_level at
all, so if we want to call mark_used uninhibited on the template
arguments from there I think we need to add a flag to mark_used that
simply disables its

  if (processing_template_decl || in_template_function ())
    return true;

early exit.  IIUC the purpose of this early exit is to avoid
instantiating a template due to a use from an uninstantiated template,
under the assumption that we'll call mark_used again at instantiation
time for that use, but in this case it seems we need to allow such
ahead-of-time instantiation since we don't reprocess the template
arguments of a non-dependent specialization again at instantiation time.

This is quite the can of worms :/

> 
> > +	{
> > +	  auto mark_used_r = [](tree *tp, int *, void *) {
> > +	    if (DECL_P (*tp))
> > +	      mark_used (*tp, tf_none);
> > +	    return NULL_TREE;
> > +	  };
> > +	  cp_walk_tree_without_duplicates (&DECL_INITIAL (arg),
> > +					   mark_used_r, nullptr);
> > +	}
> > +    }
> > +}
> > +
> >   /* We're out of SFINAE context now, so generate diagnostics for the access
> >      errors we saw earlier when instantiating D from TMPL and ARGS.  */
> >   @@ -22031,6 +22071,8 @@ instantiate_template (tree tmpl, tree orig_args,
> > tsubst_flags_t complain)
> >         push_nested_class (ctx);
> >       }
> >   +  mark_template_arguments_used (gen_tmpl, targ_ptr);
> > +
> >     tree pattern = DECL_TEMPLATE_RESULT (gen_tmpl);
> >       fndecl = NULL_TREE;
> > diff --git a/gcc/testsuite/g++.dg/template/fn-ptr3a.C
> > b/gcc/testsuite/g++.dg/template/fn-ptr3a.C
> > new file mode 100644
> > index 00000000000..7456be5d51f
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/template/fn-ptr3a.C
> > @@ -0,0 +1,25 @@
> > +// PR c++/53164
> > +// PR c++/105848
> > +// A stricter version of fn-ptr3.C that verifies using f as a template
> > +// argument alone constitutes an odr-use.
> > +
> > +template<class T>
> > +void f(T) { T::fail; } // { dg-error "fail" }
> > +
> > +template<void (*P)(int)>
> > +struct A {
> > +  static void wrap();
> > +};
> > +
> > +template<void (&P)(char)>
> > +void wrap();
> > +
> > +template<int>
> > +void g() {
> > +  A<f>::wrap(); // { dg-message "required from here" }
> > +  wrap<f>(); // { dg-message "required from here" }
> > +}
> > +
> > +int main() {
> > +  g<0>();
> > +}
> > diff --git a/gcc/testsuite/g++.dg/template/fn-ptr3b.C
> > b/gcc/testsuite/g++.dg/template/fn-ptr3b.C
> > new file mode 100644
> > index 00000000000..90d988ce896
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/template/fn-ptr3b.C
> > @@ -0,0 +1,28 @@
> > +// PR c++/53164
> > +// PR c++/105848
> > +// A C++20 version of fn-ptr3a.C using class NTTPs.
> > +// { dg-do compile { target c++20 } }
> > +
> > +template<class T>
> > +void f(T) { T::fail; } // { dg-error "fail" }
> > +
> > +struct B_int { void (*P)(int); };
> > +struct B_char { void (*P)(char); };
> > +
> > +template<B_int B>
> > +struct A {
> > +  static void wrap();
> > +};
> > +
> > +template<B_char B>
> > +void wrap();
> > +
> > +template<int>
> > +void g() {
> > +  A<B_int{f}>::wrap(); // { dg-message "required from here" }
> > +  wrap<B_char{f}>(); // { dg-message "required from here" }
> > +}
> > +
> > +int main() {
> > +  g<0>();
> > +}
> > diff --git a/gcc/testsuite/g++.dg/template/fn-ptr4.C
> > b/gcc/testsuite/g++.dg/template/fn-ptr4.C
> > new file mode 100644
> > index 00000000000..36551ec5d7f
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/template/fn-ptr4.C
> > @@ -0,0 +1,14 @@
> > +// { dg-do compile }
> > +
> > +template<void (*P)()>
> > +void wrap() {
> > +  P(); // OK, despite A::g not being accessible from here.
> > +}
> > +
> > +struct A {
> > +  static void f() {
> > +    wrap<A::g>();
> > +  }
> > +private:
> > +  static void g();
> > +};
> 
>