Re: [PATCH] c++: function NTTP argument pack considered unused [PR126280]

[Jason Merrill]

On 7/21/26 3:41 PM, Patrick Palka wrote:

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
for trunk and backports?

OK.

-- >8 --

This is just the pack version of PR c++/105848, in which our non-dependent
call pruning may cause us to not mark an otherwise unused function pointer
template argument as used.

        PR c++/126280

gcc/cp/ChangeLog:

        * pt.cc (mark_template_arguments_used): Split out loop into ...
        (mark_template_arguments_used_1): ... here. Recurse into
        argument packs.

gcc/testsuite/ChangeLog:

        * g++.dg/template/fn-ptr6.C: New test.
---
  gcc/cp/pt.cc                            | 11 +++++++++++
  gcc/testsuite/g++.dg/template/fn-ptr6.C | 26 +++++++++++++++++++++++++
  2 files changed, 37 insertions(+)
  create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr6.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 1290db45e182..ced68a238650 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -224,6 +224,7 @@ static bool uses_outer_template_parms (tree);
  static tree alias_ctad_tweaks (tree, tree);
  static tree inherited_ctad_tweaks (tree, tree, tsubst_flags_t);
  static tree deduction_guides_for (tree, bool&, tsubst_flags_t);
+static void mark_template_arguments_used_1 (tree);

/* Make the current scope suitable for access checking when we are

     processing T.  T can be FUNCTION_DECL for instantiated function
@@ -23738,6 +23739,14 @@ mark_template_arguments_used (tree tmpl, tree args)
    /* We already marked outer arguments when specializing the context.  */
    args = INNERMOST_TEMPLATE_ARGS (args);

+ mark_template_arguments_used_1 (args);

+}
+
+/* Main recursive part of the above.  */
+
+static void
+mark_template_arguments_used_1 (tree args)
+{
    for (tree arg : tree_vec_range (args))
      {
        /* A (pointer/reference to) function or variable NTTP argument.  */
@@ -23778,6 +23787,8 @@ mark_template_arguments_used (tree tmpl, tree args)
          cp_walk_tree_without_duplicates (&DECL_INITIAL (arg),
                                           mark_used_r, nullptr);
        }
+      else if (TREE_CODE (arg) == NONTYPE_ARGUMENT_PACK)
+       mark_template_arguments_used_1 (ARGUMENT_PACK_ARGS (arg));
      }
  }

diff --git a/gcc/testsuite/g++.dg/template/fn-ptr6.C b/gcc/testsuite/g++.dg/template/fn-ptr6.C

new file mode 100644
index 000000000000..fa111c6d9429
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/fn-ptr6.C
@@ -0,0 +1,26 @@
+// PR c++/126280
+// A version of fn-ptr3a.C where the template parameter is a pack.
+// { dg-do compile { target c++11 } }
+
+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>();
+}