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

[Patrick Palka <ppalka@redhat.com>]

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.
> 
> 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);
+      }
+}
+
 /* 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 +22054,8 @@ 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-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();
+};
-- 
2.40.0.130.g27d43aaaf5