[gcc r13-10474] c++: fix propagating REF_PARENTHESIZED_P [PR116379]

https://gcc.gnu.org/g:385482a2d70c13f2741f0fede95de93eefd63f1d

commit r13-10474-g385482a2d70c13f2741f0fede95de93eefd63f1d
Author: Marek Polacek <[email protected]>
Date:   Thu Aug 20 14:59:46 2026 -0400

    c++: fix propagating REF_PARENTHESIZED_P [PR116379]
    
    Here we have:
    
      template<typename T>
      struct X{
          T val;
          decltype(auto) value(){
              return (val);
          }
      };
    
    where the return type of value should be 'int &' since '(val)' is an
    expression, not a name, and decltype(auto) performs the type deduction
    using the decltype rules.
    
    The problem is that we weren't propagating REF_PARENTHESIZED_P
    correctly: the return value of finish_non_static_data_member in this
    test was a REFERENCE_REF_P, so we didn't set the flag.  We should
    use force_paren_expr like below.
    
            PR c++/116379
    
    gcc/cp/ChangeLog:
    
            * pt.cc (tsubst_expr) <COMPONENT_REF>: Use force_paren_expr to set
            REF_PARENTHESIZED_P.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp1y/decltype-auto9.C: New test.
    
    Reviewed-by: Jason Merrill <[email protected]>

Diff:
---
 gcc/cp/pt.cc                                |  4 ++--
 gcc/testsuite/g++.dg/cpp1y/decltype-auto9.C | 15 +++++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 9159c043cdc6..b01284ac12dd 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -21538,8 +21538,8 @@ tsubst_copy_and_build (tree t,
 	    r = finish_non_static_data_member (member, object, NULL_TREE,
 					       complain);
 	    pop_deferring_access_checks ();
-	    if (TREE_CODE (r) == COMPONENT_REF)
-	      REF_PARENTHESIZED_P (r) = REF_PARENTHESIZED_P (t);
+	    if (REF_PARENTHESIZED_P (t))
+	      r = force_paren_expr (r);
 	    RETURN (r);
 	  }
 	else if (type_dependent_expression_p (object))
diff --git a/gcc/testsuite/g++.dg/cpp1y/decltype-auto9.C b/gcc/testsuite/g++.dg/cpp1y/decltype-auto9.C
new file mode 100644
index 000000000000..1ccf95a01702
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/decltype-auto9.C
@@ -0,0 +1,15 @@
+// PR c++/116379
+// { dg-do compile { target c++14 } }
+
+template<typename T>
+struct X {
+  T val;
+  decltype(auto) value() { return (val); }
+};
+
+int main() {
+  int i = 0;
+  X<int&&> x{ static_cast<int&&>(i) };
+  using type = decltype(x.value());
+  using type = int&;
+}