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

[Jason Merrill <jason@redhat.com>]

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.

> +	}
> +      /* 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)?

> +	{
> +	  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();
> +};