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

[Jason Merrill <jason@redhat.com>]

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.

> This is what I have so far, it's imperfect but perhaps good enough?
> bootstrapped and regtested on x86_64-pc-linux-gnu:
> 
> -- >8 --
> 
> Subject: [PATCH] c++: improve "NTTP argument considered unused" fix [PR53164,
>   PR105848]
> 
> 	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                             | 88 +++++++++++++++++++-----
>   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, 137 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..83902b0be8d 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 +21895,63 @@ 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 (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, 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 (VAR_OR_FUNCTION_DECL_P (arg))
> +	    {
> +	      /* Pass tf_none to avoid duplicate diagnostics: the call
> +		 ought to succeed without error, or a previous call
> +		 must have already issued an error.  */
> +	      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.  */
>   
> @@ -22031,6 +22081,8 @@ 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();
> +};