From: Nathaniel Shead <nathanieloshead@gmail.com>
Date: Sun, 5 Oct 2025 10:30:56 +0000 (+1100)
Subject: c++: Ensure mark_lvalue_use returns an lvalue [PR122163]
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ac524299110838ed8eafb615d100a28649af7f18;p=thirdparty%2Fgcc.git

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

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.

	PR c++/122163

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 <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>
---

diff --git a/gcc/cp/expr.cc b/gcc/cp/expr.cc
index f2e99397aca..a29cb2461a5 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);
+		}
+	      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();
+}