[gcc r16-9528] c++/reflection: Handle stale parameter decarations in eval_is_explicit_object_parameter

https://gcc.gnu.org/g:4d671ef9247d1752bce4f0edcf0a78b4ef82c46f

commit r16-9528-g4d671ef9247d1752bce4f0edcf0a78b4ef82c46f
Author: Wang Jinghao <[email protected]>
Date:   Wed Jun 17 11:17:47 2026 -0400

    c++/reflection: Handle stale parameter decarations in eval_is_explicit_object_parameter
    
    eval_is_explicit_object_parameter() determines whether a parameter is
    an explicit object parameter by checking whether the passed-in tree
    node is the first parameter in its parameter chain. As described in
    the comment for maybe_update_function_parm(), if the sequence is
    declaration -> reflection -> definition, the old PARM_DECL will be
    compared against the new PARM_DECL, causing the check to produce an
    incorrect result.
    Calling maybe_update_function_parm() before the comparison updates
    the old reflection to the new PARM_DECL in the chain, thereby
    avoiding this error.
    
    gcc/cp/ChangeLog:
    
            * reflect.cc (eval_is_explicit_object_parameter): Call
            maybe_update_function_parm before checking the parameter
            against DECL_ARGUMENTS.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/reflect/is_explicit_object_parameter2.C: New test.
    
    Signed-off-by: Wang Jinghao <[email protected]>
    Reviewed-by: Patrick Palka <[email protected]>
    (cherry picked from commit 06429c88ecd5de5b597af3686e521b4e2c106588)

Diff:
---
 gcc/cp/reflect.cc                                       | 10 ++++++----
 .../g++.dg/reflect/is_explicit_object_parameter2.C      | 17 +++++++++++++++++
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/gcc/cp/reflect.cc b/gcc/cp/reflect.cc
index 66596fcb15e2..e9b0ab77eca3 100644
--- a/gcc/cp/reflect.cc
+++ b/gcc/cp/reflect.cc
@@ -1828,11 +1828,13 @@ eval_is_data_member_spec (const_tree r, reflect_kind kind)
    object parameter.  Otherwise, false.  */
 
 static tree
-eval_is_explicit_object_parameter (const_tree r, reflect_kind kind)
+eval_is_explicit_object_parameter (tree r, reflect_kind kind)
 {
-  if (eval_is_function_parameter (r, kind) == boolean_true_node
-      && r == DECL_ARGUMENTS (DECL_CONTEXT (r))
-      && DECL_XOBJ_MEMBER_FUNCTION_P (DECL_CONTEXT (r)))
+  if (eval_is_function_parameter (r, kind) == boolean_false_node)
+    return boolean_false_node;
+  r = maybe_update_function_parm (r);
+  tree fn = DECL_CONTEXT (r);
+  if (r == DECL_ARGUMENTS (fn) && DECL_XOBJ_MEMBER_FUNCTION_P (fn))
     return boolean_true_node;
   else
     return boolean_false_node;
diff --git a/gcc/testsuite/g++.dg/reflect/is_explicit_object_parameter2.C b/gcc/testsuite/g++.dg/reflect/is_explicit_object_parameter2.C
new file mode 100644
index 000000000000..cc24e439bc81
--- /dev/null
+++ b/gcc/testsuite/g++.dg/reflect/is_explicit_object_parameter2.C
@@ -0,0 +1,17 @@
+// { dg-do compile { target c++26 } }
+// { dg-additional-options "-freflection" }
+// Test std::meta::is_explicit_object_parameter.
+
+#include <meta>
+
+using namespace std::meta;
+
+struct S {
+  void f (this S, int);
+};
+
+constexpr auto p = parameters_of (^^S::f)[0];
+
+void S::f (this S, int) {}
+
+static_assert (is_explicit_object_parameter (p));