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

[Jason Merrill <jason@redhat.com>]

On 3/31/23 15:06, Patrick Palka wrote:
> On Fri, 31 Mar 2023, Jason Merrill wrote:
> 
>> On 3/31/23 11:55, Patrick Palka wrote:
>>> On Fri, 31 Mar 2023, Patrick Palka wrote:
>>>
>>>> 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 :/
>>
>> Yes, I think I'd prefer to move toward the
>> instantiate_class_template/instantiate_body consistency, to avoid unnecessary
>> marking in uninstantiated templates.
> Aha, makes sense, that approach seems to align best with our existing
> efforts to avoid instantiation from uninstantiated templates.  Like so?
> Bootstrapped and regtested on x86_64-pc-linux-gnu.

OK.

> -- >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're really passing 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 instantiation
> 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 be large and thus expensive to walk repeatedly.  The change to
> invalid_tparm_referent_p is needed to avoid incorrectly rejecting class
> NTTP arguments containing function pointers as in the testcase.
> 
> (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_body): 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                             | 86 +++++++++++++++++++-----
>   gcc/testsuite/g++.dg/template/fn-ptr3a.C | 27 ++++++++
>   gcc/testsuite/g++.dg/template/fn-ptr3b.C | 30 +++++++++
>   gcc/testsuite/g++.dg/template/fn-ptr4.C  | 14 ++++
>   4 files changed, 139 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 dd7f0db9658..022f295b36f 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 +21895,61 @@ 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 an instantiation of TMPL, to ensure that each such entity is
> +   considered odr-used (and therefore marked for instantiation) regardless of
> +   whether the specialization was first formed in a template context (which
> +   inhibits mark_used).
> +
> +   This function assumes push_to_top_level has been called beforehand.  */
> +
> +static void
> +mark_template_arguments_used (tree tmpl, tree args)
> +{
> +  /* It suffices to do this only when instantiating a primary template.  */
> +  if (TREE_CODE (tmpl) != 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 (VAR_OR_FUNCTION_DECL_P (arg))
> +	    {
> +	      /* Pass tf_none to avoid duplicate diagnostics: if this call
> +		 fails then an earlier call to mark_used for this argument
> +		 must have also failed and emitted a diagnostic.  */
> +	      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))
> +	{
> +	  auto mark_used_r = [](tree *tp, int *, void *) {
> +	    if (VAR_OR_FUNCTION_DECL_P (*tp))
> +	      {
> +		bool ok = mark_used (*tp, tf_none);
> +		gcc_checking_assert (ok || seen_error ());
> +	      }
> +	    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.  */
>   
> @@ -26775,6 +26823,8 @@ instantiate_body (tree pattern, tree args, tree d, bool nested_p)
>         c_inhibit_evaluation_warnings = 0;
>       }
>   
> +  mark_template_arguments_used (pattern, args);
> +
>     if (VAR_P (d))
>       {
>         /* The variable might be a lambda's extra scope, and that
> 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..345c621a0c6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/fn-ptr3a.C
> @@ -0,0 +1,27 @@
> +// 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 {
> +  // P not called
> +};
> +
> +template<void (&P)(char)>
> +void wrap() {
> +  // P not called
> +}
> +
> +template<int>
> +void g() {
> +  A<f> a; // { dg-message "required from" }
> +  wrap<f>(); // { dg-message "required from" }
> +}
> +
> +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..899c355fb38
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/fn-ptr3b.C
> @@ -0,0 +1,30 @@
> +// 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 {
> +  // B.P not called
> +};
> +
> +template<B_char B>
> +void wrap() {
> +  // B.P not called
> +}
> +
> +template<int>
> +void g() {
> +  A<B_int{f}> a; // { dg-message "required from" }
> +  wrap<B_char{f}>(); // { dg-message "required from" }
> +}
> +
> +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();
> +};