[gcc r16-5489] c++: make __reference_*_from_temporary honor access [PR120529]

[Marek Polacek]

https://gcc.gnu.org/g:e82a8e51848b5a74edb530f7597861f7080662c3

commit r16-5489-ge82a8e51848b5a74edb530f7597861f7080662c3
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Nov 20 13:57:43 2025 -0500

    c++: make __reference_*_from_temporary honor access [PR120529]
    
    This PR reports that our __reference_*_from_temporary ignore access
    control.  The reason is that we only check if implicit_conversion
    works, but not if the conversion can actually be performed, via
    convert_like.
    
            PR c++/120529
    
    gcc/cp/ChangeLog:
    
            * call.cc (ref_conv_binds_to_temporary): Don't ignore access control.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/ext/reference_xes_from_temporary1.C: New test.
    
    Reviewed-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/call.cc                                     | 16 +++++++++----
 .../g++.dg/ext/reference_xes_from_temporary1.C     | 28 ++++++++++++++++++++++
 2 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index f80d597b3394..ea89130572b8 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -10203,11 +10203,19 @@ ref_conv_binds_to_temporary (tree type, tree expr, bool direct_init_p/*=false*/)
   const int flags = direct_init_p ? LOOKUP_NORMAL : LOOKUP_IMPLICIT;
   conversion *conv = implicit_conversion (type, TREE_TYPE (expr), expr,
 					  /*c_cast_p=*/false, flags, tf_none);
-  tristate ret (tristate::TS_UNKNOWN);
-  if (conv && !conv->bad_p)
-    ret = tristate (conv_binds_ref_to_temporary (conv));
+  if (!conv || conv->bad_p)
+    return tristate::unknown ();
 
-  return ret;
+  if (conv_binds_ref_to_temporary (conv))
+    {
+      /* Actually perform the conversion to check access control.  */
+      if (convert_like (conv, expr, tf_none) != error_mark_node)
+	return tristate (true);
+      else
+	return tristate::unknown ();
+    }
+
+  return tristate (false);
 }
 
 /* Call the trivial destructor for INSTANCE, which can be either an lvalue of
diff --git a/gcc/testsuite/g++.dg/ext/reference_xes_from_temporary1.C b/gcc/testsuite/g++.dg/ext/reference_xes_from_temporary1.C
new file mode 100644
index 000000000000..274de05e7b7a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/reference_xes_from_temporary1.C
@@ -0,0 +1,28 @@
+// PR c++/120529
+// { dg-do compile { target c++11 } }
+
+#define SA(X) static_assert((X),#X)
+
+class Dst {};
+
+class Src {
+private:
+    operator Dst() const;
+};
+
+class Src2 {
+protected:
+    operator Dst() const;
+};
+
+class Src3 {
+public:
+    operator Dst() const;
+};
+
+SA (!__reference_converts_from_temporary (Dst&&, Src));
+SA (!__reference_constructs_from_temporary (Dst&&, Src));
+SA (!__reference_converts_from_temporary (Dst&&, Src2));
+SA (!__reference_constructs_from_temporary (Dst&&, Src2));
+SA (__reference_converts_from_temporary (Dst&&, Src3));
+SA (__reference_constructs_from_temporary (Dst&&, Src3));