Re: [PATCH] c++: Ensure mark_lvalue_use returns an lvalue [PR122163]

[Jason Merrill]

On 10/5/25 11:34 AM, Nathaniel Shead wrote:
Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?
-- >8 --

When processing a tentative capture of a rvalue reference, mark_use
folds it away to the referred-to entity.  But this is an rvalue, and
when called from an lvalue context an rvalue reference should still be
an lvalue.

gcc/cp/ChangeLog:

        * expr.cc (mark_use): When processing a reference, always return
        an lvalue reference when !rvalue_p.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp0x/lambda/lambda-ref3.C: New test.

Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
---
  gcc/cp/expr.cc                                 | 11 ++++++++++-
  .../g++.dg/cpp0x/lambda/lambda-ref3.C          | 18 ++++++++++++++++++
  2 files changed, 28 insertions(+), 1 deletion(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-ref3.C

diff --git a/gcc/cp/expr.cc b/gcc/cp/expr.cc
index 32dc3eee78f..092cf886a25 100644
--- a/gcc/cp/expr.cc
+++ b/gcc/cp/expr.cc
@@ -183,7 +183,16 @@ mark_use (tree expr, bool rvalue_p, bool read_p,
            }
          tree r = mark_rvalue_use (ref, loc, reject_builtin);
          if (r != ref)
-           expr = convert_from_reference (r);
+           {
+             if (!rvalue_p)
+               {
+                 /* Make sure we still return an lvalue.  */
+                 gcc_assert (TREE_CODE (r) == NOP_EXPR);
+                 TREE_TYPE (r) = cp_build_reference_type (TREE_TYPE (r),
+                                                          false);

Perhaps we could generalize the is_normal_capture_proxy block above to 
handle !rvalue_p, to return the captured variable directly with no 
NOP_EXPR/ADDR_EXPR?

But the patch is OK as is if you'd rather not bother.

+               }
+             expr = convert_from_reference (r);
+           }
        }
        break;
  
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-ref3.C b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-ref3.C
new file mode 100644
index 00000000000..1e085b23796
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-ref3.C
@@ -0,0 +1,18 @@
+// { dg-do run { target c++11 } }
+
+int x;
+int main() {
+  constexpr int&& r = static_cast<int&&>(x);
+  r = 1;
+
+  auto a = [] { r = 2; };  // Not an ODR-use of 'r' so no capture needed.
+  auto b = [&] { r = 3; };
+
+  a();
+  if (r != 2)
+    __builtin_abort();
+
+  b();
+  if (r != 3)
+    __builtin_abort();
+}