[gcc r14-11780] c++: fix propagating REF_PARENTHESIZED_P [PR116379]

[Marek Polacek via Gcc-cvs]

https://gcc.gnu.org/g:c11cb308c1e2aae427d6f32a1144ae8907dc2649

commit r14-11780-gc11cb308c1e2aae427d6f32a1144ae8907dc2649
Author: Marek Polacek <pola...@redhat.com>
Date:   Wed May 14 10:40:54 2025 -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 <ja...@redhat.com>

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 b6a87b0f5c20..65fa85b06100 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -21277,8 +21277,8 @@ tsubst_expr (tree t, tree args, tsubst_flags_t 
complain, tree in_decl)
          {
            r = finish_non_static_data_member (member, object, NULL_TREE,
                                               complain);
-           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&;
+}