[gcc r16-9409] c++: function NTTP argument in pack considered unused [PR126280]

https://gcc.gnu.org/g:9fad76fa86f5a1464fbe52f926750a389727910d

commit r16-9409-g9fad76fa86f5a1464fbe52f926750a389727910d
Author: Patrick Palka <[email protected]>
Date:   Sat Jul 25 16:12:19 2026 -0400

    c++: function NTTP argument in pack considered unused [PR126280]
    
    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 within a pack 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.
    
    Reviewed-by: Jason Merrill <[email protected]>
    (cherry picked from commit d47f48643b3db4e207b0cc3382f614aa76c9c2c5)

Diff:
---
 gcc/cp/pt.cc                            | 11 +++++++++++
 gcc/testsuite/g++.dg/template/fn-ptr6.C | 26 ++++++++++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index f055cc4e276b..9d3a972e3002 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -223,6 +223,7 @@ 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
@@ -23538,6 +23539,14 @@ 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.  */
@@ -23578,6 +23587,8 @@ 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>();
+}